Age Owner Branch data TLA Line data Source code
1 : : /*-------------------------------------------------------------------------
2 : : *
3 : : * lwlock.c
4 : : * Lightweight lock manager
5 : : *
6 : : * Lightweight locks are intended primarily to provide mutual exclusion of
7 : : * access to shared-memory data structures. Therefore, they offer both
8 : : * exclusive and shared lock modes (to support read/write and read-only
9 : : * access to a shared object). There are few other frammishes. User-level
10 : : * locking should be done with the full lock manager --- which depends on
11 : : * LWLocks to protect its shared state.
12 : : *
13 : : * In addition to exclusive and shared modes, lightweight locks can be used to
14 : : * wait until a variable changes value. The variable is initially not set
15 : : * when the lock is acquired with LWLockAcquire, i.e. it remains set to the
16 : : * value it was set to when the lock was released last, and can be updated
17 : : * without releasing the lock by calling LWLockUpdateVar. LWLockWaitForVar
18 : : * waits for the variable to be updated, or until the lock is free. When
19 : : * releasing the lock with LWLockReleaseClearVar() the value can be set to an
20 : : * appropriate value for a free lock. The meaning of the variable is up to
21 : : * the caller, the lightweight lock code just assigns and compares it.
22 : : *
23 : : * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
24 : : * Portions Copyright (c) 1994, Regents of the University of California
25 : : *
26 : : * IDENTIFICATION
27 : : * src/backend/storage/lmgr/lwlock.c
28 : : *
29 : : * NOTES:
30 : : *
31 : : * This used to be a pretty straight forward reader-writer lock
32 : : * implementation, in which the internal state was protected by a
33 : : * spinlock. Unfortunately the overhead of taking the spinlock proved to be
34 : : * too high for workloads/locks that were taken in shared mode very
35 : : * frequently. Often we were spinning in the (obviously exclusive) spinlock,
36 : : * while trying to acquire a shared lock that was actually free.
37 : : *
38 : : * Thus a new implementation was devised that provides wait-free shared lock
39 : : * acquisition for locks that aren't exclusively locked.
40 : : *
41 : : * The basic idea is to have a single atomic variable 'lockcount' instead of
42 : : * the formerly separate shared and exclusive counters and to use atomic
43 : : * operations to acquire the lock. That's fairly easy to do for plain
44 : : * rw-spinlocks, but a lot harder for something like LWLocks that want to wait
45 : : * in the OS.
46 : : *
47 : : * For lock acquisition we use an atomic compare-and-exchange on the lockcount
48 : : * variable. For exclusive lock we swap in a sentinel value
49 : : * (LW_VAL_EXCLUSIVE), for shared locks we count the number of holders.
50 : : *
51 : : * To release the lock we use an atomic decrement to release the lock. If the
52 : : * new value is zero (we get that atomically), we know we can/have to release
53 : : * waiters.
54 : : *
55 : : * Obviously it is important that the sentinel value for exclusive locks
56 : : * doesn't conflict with the maximum number of possible share lockers -
57 : : * luckily MAX_BACKENDS makes that easily possible.
58 : : *
59 : : *
60 : : * The attentive reader might have noticed that naively doing the above has a
61 : : * glaring race condition: We try to lock using the atomic operations and
62 : : * notice that we have to wait. Unfortunately by the time we have finished
63 : : * queuing, the former locker very well might have already finished its
64 : : * work. That's problematic because we're now stuck waiting inside the OS.
65 : :
66 : : * To mitigate those races we use a two phased attempt at locking:
67 : : * Phase 1: Try to do it atomically, if we succeed, nice
68 : : * Phase 2: Add ourselves to the waitqueue of the lock
69 : : * Phase 3: Try to grab the lock again, if we succeed, remove ourselves from
70 : : * the queue
71 : : * Phase 4: Sleep till wake-up, goto Phase 1
72 : : *
73 : : * This protects us against the problem from above as nobody can release too
74 : : * quick, before we're queued, since after Phase 2 we're already queued.
75 : : * -------------------------------------------------------------------------
76 : : */
77 : : #include "postgres.h"
78 : :
79 : : #include "miscadmin.h"
80 : : #include "pg_trace.h"
81 : : #include "pgstat.h"
82 : : #include "port/pg_bitutils.h"
83 : : #include "storage/proc.h"
84 : : #include "storage/proclist.h"
85 : : #include "storage/procnumber.h"
86 : : #include "storage/spin.h"
87 : : #include "storage/subsystems.h"
88 : : #include "utils/memutils.h"
89 : : #include "utils/wait_event.h"
90 : :
91 : : #ifdef LWLOCK_STATS
92 : : #include "utils/hsearch.h"
93 : : #endif
94 : :
95 : :
96 : : #define LW_FLAG_HAS_WAITERS ((uint32) 1 << 31)
97 : : #define LW_FLAG_WAKE_IN_PROGRESS ((uint32) 1 << 30)
98 : : #define LW_FLAG_LOCKED ((uint32) 1 << 29)
99 : : #define LW_FLAG_BITS 3
100 : : #define LW_FLAG_MASK (((1<<LW_FLAG_BITS)-1)<<(32-LW_FLAG_BITS))
101 : :
102 : : /* assumes MAX_BACKENDS is a (power of 2) - 1, checked below */
103 : : #define LW_VAL_EXCLUSIVE (MAX_BACKENDS + 1)
104 : : #define LW_VAL_SHARED 1
105 : :
106 : : /* already (power of 2)-1, i.e. suitable for a mask */
107 : : #define LW_SHARED_MASK MAX_BACKENDS
108 : : #define LW_LOCK_MASK (MAX_BACKENDS | LW_VAL_EXCLUSIVE)
109 : :
110 : :
111 : : StaticAssertDecl(((MAX_BACKENDS + 1) & MAX_BACKENDS) == 0,
112 : : "MAX_BACKENDS + 1 needs to be a power of 2");
113 : :
114 : : StaticAssertDecl((MAX_BACKENDS & LW_FLAG_MASK) == 0,
115 : : "MAX_BACKENDS and LW_FLAG_MASK overlap");
116 : :
117 : : StaticAssertDecl((LW_VAL_EXCLUSIVE & LW_FLAG_MASK) == 0,
118 : : "LW_VAL_EXCLUSIVE and LW_FLAG_MASK overlap");
119 : :
120 : : /*
121 : : * There are three sorts of LWLock "tranches":
122 : : *
123 : : * 1. The individually-named locks defined in lwlocklist.h each have their
124 : : * own tranche. We absorb the names of these tranches from there into
125 : : * BuiltinTrancheNames here.
126 : : *
127 : : * 2. There are some predefined tranches for built-in groups of locks defined
128 : : * in lwlocklist.h. We absorb the names of these tranches, too.
129 : : *
130 : : * 3. Extensions can create new tranches, via either RequestNamedLWLockTranche
131 : : * or LWLockNewTrancheId. These are stored in shared memory and can be
132 : : * accessed via LWLockTranches.
133 : : *
134 : : * All these names are user-visible as wait event names, so choose with care
135 : : * ... and do not forget to update the documentation's list of wait events.
136 : : */
137 : : static const char *const BuiltinTrancheNames[] = {
138 : : #define PG_LWLOCK(id, lockname) [id] = CppAsString(lockname),
139 : : #define PG_LWLOCKTRANCHE(id, lockname) [LWTRANCHE_##id] = CppAsString(lockname),
140 : : #include "storage/lwlocklist.h"
141 : : #undef PG_LWLOCK
142 : : #undef PG_LWLOCKTRANCHE
143 : : };
144 : :
145 : : StaticAssertDecl(lengthof(BuiltinTrancheNames) ==
146 : : LWTRANCHE_FIRST_USER_DEFINED,
147 : : "missing entries in BuiltinTrancheNames[]");
148 : :
149 : : /* Main array of LWLocks in shared memory */
150 : : LWLockPadded *MainLWLockArray = NULL;
151 : :
152 : : /*
153 : : * We use this structure to keep track of locked LWLocks for release
154 : : * during error recovery. Normally, only a few will be held at once, but
155 : : * occasionally the number can be much higher.
156 : : */
157 : : #define MAX_SIMUL_LWLOCKS 200
158 : :
159 : : /* struct representing the LWLocks we're holding */
160 : : typedef struct LWLockHandle
161 : : {
162 : : LWLock *lock;
163 : : LWLockMode mode;
164 : : } LWLockHandle;
165 : :
166 : : static int num_held_lwlocks = 0;
167 : : static LWLockHandle held_lwlocks[MAX_SIMUL_LWLOCKS];
168 : :
169 : : /* Maximum number of LWLock tranches that can be assigned by extensions */
170 : : #define MAX_USER_DEFINED_TRANCHES 256
171 : :
172 : : /*
173 : : * Shared memory structure holding user-defined tranches.
174 : : */
175 : : typedef struct LWLockTrancheShmemData
176 : : {
177 : : /* This is indexed by tranche ID minus LWTRANCHE_FIRST_USER_DEFINED */
178 : : struct
179 : : {
180 : : char name[NAMEDATALEN];
181 : :
182 : : /*
183 : : * Index of the tranche's locks in MainLWLockArray if this tranche was
184 : : * allocated with RequestNamedLWLockTranche(), or -1 if the tranche
185 : : * was allocated with LWLockNewTrancheId()
186 : : */
187 : : int main_array_idx;
188 : : } user_defined[MAX_USER_DEFINED_TRANCHES];
189 : :
190 : : int num_user_defined; /* 'user_defined' entries in use */
191 : :
192 : : slock_t lock; /* protects the above */
193 : : } LWLockTrancheShmemData;
194 : :
195 : : static LWLockTrancheShmemData *LWLockTranches;
196 : :
197 : : /* backend-local copy of LWLockTranches->num_user_defined */
198 : : static int LocalNumUserDefinedTranches;
199 : :
200 : : /*
201 : : * NamedLWLockTrancheRequests is a list of tranches requested with
202 : : * RequestNamedLWLockTranche(). It is only valid in the postmaster; after
203 : : * startup the tranches are tracked in LWLockTranches in shared memory.
204 : : */
205 : : typedef struct NamedLWLockTrancheRequest
206 : : {
207 : : char tranche_name[NAMEDATALEN];
208 : : int num_lwlocks;
209 : : } NamedLWLockTrancheRequest;
210 : :
211 : : static List *NamedLWLockTrancheRequests = NIL;
212 : :
213 : : /* Size of MainLWLockArray. Only valid in postmaster. */
214 : : static int num_main_array_locks;
215 : :
216 : : static void LWLockShmemRequest(void *arg);
217 : : static void LWLockShmemInit(void *arg);
218 : :
219 : : const ShmemCallbacks LWLockCallbacks = {
220 : : .request_fn = LWLockShmemRequest,
221 : : .init_fn = LWLockShmemInit,
222 : : };
223 : :
224 : :
225 : : static inline void LWLockReportWaitStart(LWLock *lock);
226 : : static inline void LWLockReportWaitEnd(void);
227 : : static const char *GetLWTrancheName(uint16 trancheId);
228 : :
229 : : #define T_NAME(lock) \
230 : : GetLWTrancheName((lock)->tranche)
231 : :
232 : : #ifdef LWLOCK_STATS
233 : : typedef struct lwlock_stats_key
234 : : {
235 : : int tranche;
236 : : void *instance;
237 : : } lwlock_stats_key;
238 : :
239 : : typedef struct lwlock_stats
240 : : {
241 : : lwlock_stats_key key;
242 : : int sh_acquire_count;
243 : : int ex_acquire_count;
244 : : int block_count;
245 : : int dequeue_self_count;
246 : : int spin_delay_count;
247 : : } lwlock_stats;
248 : :
249 : : static HTAB *lwlock_stats_htab;
250 : : static lwlock_stats lwlock_stats_dummy;
251 : : #endif
252 : :
253 : : #ifdef LOCK_DEBUG
254 : : bool Trace_lwlocks = false;
255 : :
256 : : inline static void
257 : : PRINT_LWDEBUG(const char *where, LWLock *lock, LWLockMode mode)
258 : : {
259 : : /* hide statement & context here, otherwise the log is just too verbose */
260 : : if (Trace_lwlocks)
261 : : {
262 : : uint32 state = pg_atomic_read_u32(&lock->state);
263 : :
264 : : ereport(LOG,
265 : : (errhidestmt(true),
266 : : errhidecontext(true),
267 : : errmsg_internal("%d: %s(%s %p): excl %u shared %u haswaiters %u waiters %u waking %d",
268 : : MyProcPid,
269 : : where, T_NAME(lock), lock,
270 : : (state & LW_VAL_EXCLUSIVE) != 0,
271 : : state & LW_SHARED_MASK,
272 : : (state & LW_FLAG_HAS_WAITERS) != 0,
273 : : pg_atomic_read_u32(&lock->nwaiters),
274 : : (state & LW_FLAG_WAKE_IN_PROGRESS) != 0)));
275 : : }
276 : : }
277 : :
278 : : inline static void
279 : : LOG_LWDEBUG(const char *where, LWLock *lock, const char *msg)
280 : : {
281 : : /* hide statement & context here, otherwise the log is just too verbose */
282 : : if (Trace_lwlocks)
283 : : {
284 : : ereport(LOG,
285 : : (errhidestmt(true),
286 : : errhidecontext(true),
287 : : errmsg_internal("%s(%s %p): %s", where,
288 : : T_NAME(lock), lock, msg)));
289 : : }
290 : : }
291 : :
292 : : #else /* not LOCK_DEBUG */
293 : : #define PRINT_LWDEBUG(a,b,c) ((void)0)
294 : : #define LOG_LWDEBUG(a,b,c) ((void)0)
295 : : #endif /* LOCK_DEBUG */
296 : :
297 : : #ifdef LWLOCK_STATS
298 : :
299 : : static void init_lwlock_stats(void);
300 : : static void print_lwlock_stats(int code, Datum arg);
301 : : static lwlock_stats * get_lwlock_stats_entry(LWLock *lock);
302 : :
303 : : static void
304 : : init_lwlock_stats(void)
305 : : {
306 : : HASHCTL ctl;
307 : : static MemoryContext lwlock_stats_cxt = NULL;
308 : : static bool exit_registered = false;
309 : :
310 : : if (lwlock_stats_cxt != NULL)
311 : : MemoryContextDelete(lwlock_stats_cxt);
312 : :
313 : : /*
314 : : * The LWLock stats will be updated within a critical section, which
315 : : * requires allocating new hash entries. Allocations within a critical
316 : : * section are normally not allowed because running out of memory would
317 : : * lead to a PANIC, but LWLOCK_STATS is debugging code that's not normally
318 : : * turned on in production, so that's an acceptable risk. The hash entries
319 : : * are small, so the risk of running out of memory is minimal in practice.
320 : : */
321 : : lwlock_stats_cxt = AllocSetContextCreate(TopMemoryContext,
322 : : "LWLock stats",
323 : : ALLOCSET_DEFAULT_SIZES);
324 : : MemoryContextAllowInCriticalSection(lwlock_stats_cxt, true);
325 : :
326 : : ctl.keysize = sizeof(lwlock_stats_key);
327 : : ctl.entrysize = sizeof(lwlock_stats);
328 : : ctl.hcxt = lwlock_stats_cxt;
329 : : lwlock_stats_htab = hash_create("lwlock stats", 16384, &ctl,
330 : : HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
331 : : if (!exit_registered)
332 : : {
333 : : on_shmem_exit(print_lwlock_stats, 0);
334 : : exit_registered = true;
335 : : }
336 : : }
337 : :
338 : : static void
339 : : print_lwlock_stats(int code, Datum arg)
340 : : {
341 : : HASH_SEQ_STATUS scan;
342 : : lwlock_stats *lwstats;
343 : :
344 : : hash_seq_init(&scan, lwlock_stats_htab);
345 : :
346 : : /* Grab an LWLock to keep different backends from mixing reports */
347 : : LWLockAcquire(&MainLWLockArray[0].lock, LW_EXCLUSIVE);
348 : :
349 : : while ((lwstats = (lwlock_stats *) hash_seq_search(&scan)) != NULL)
350 : : {
351 : : fprintf(stderr,
352 : : "PID %d lwlock %s %p: shacq %u exacq %u blk %u spindelay %u dequeue self %u\n",
353 : : MyProcPid, GetLWTrancheName(lwstats->key.tranche),
354 : : lwstats->key.instance, lwstats->sh_acquire_count,
355 : : lwstats->ex_acquire_count, lwstats->block_count,
356 : : lwstats->spin_delay_count, lwstats->dequeue_self_count);
357 : : }
358 : :
359 : : LWLockRelease(&MainLWLockArray[0].lock);
360 : : }
361 : :
362 : : static lwlock_stats *
363 : : get_lwlock_stats_entry(LWLock *lock)
364 : : {
365 : : lwlock_stats_key key;
366 : : lwlock_stats *lwstats;
367 : : bool found;
368 : :
369 : : /*
370 : : * During shared memory initialization, the hash table doesn't exist yet.
371 : : * Stats of that phase aren't very interesting, so just collect operations
372 : : * on all locks in a single dummy entry.
373 : : */
374 : : if (lwlock_stats_htab == NULL)
375 : : return &lwlock_stats_dummy;
376 : :
377 : : /* Fetch or create the entry. */
378 : : MemSet(&key, 0, sizeof(key));
379 : : key.tranche = lock->tranche;
380 : : key.instance = lock;
381 : : lwstats = hash_search(lwlock_stats_htab, &key, HASH_ENTER, &found);
382 : : if (!found)
383 : : {
384 : : lwstats->sh_acquire_count = 0;
385 : : lwstats->ex_acquire_count = 0;
386 : : lwstats->block_count = 0;
387 : : lwstats->dequeue_self_count = 0;
388 : : lwstats->spin_delay_count = 0;
389 : : }
390 : : return lwstats;
391 : : }
392 : : #endif /* LWLOCK_STATS */
393 : :
394 : :
395 : : /*
396 : : * Compute number of LWLocks required by user-defined tranches requested with
397 : : * RequestNamedLWLockTranche(). These will be allocated in the main array.
398 : : */
399 : : static int
2182 tgl@sss.pgh.pa.us 400 :CBC 1244 : NumLWLocksForNamedTranches(void)
401 : : {
3743 rhaas@postgresql.org 402 : 1244 : int numLocks = 0;
403 : :
40 heikki.linnakangas@i 404 [ + + + + :GNC 2490 : foreach_ptr(NamedLWLockTrancheRequest, request, NamedLWLockTrancheRequests)
+ + ]
405 : : {
406 : 2 : numLocks += request->num_lwlocks;
407 : : }
408 : :
3743 rhaas@postgresql.org 409 :CBC 1244 : return numLocks;
410 : : }
411 : :
412 : : /*
413 : : * Request shmem space for user-defined tranches and the main LWLock array.
414 : : */
415 : : static void
29 heikki.linnakangas@i 416 :GNC 1244 : LWLockShmemRequest(void *arg)
417 : : {
418 : : size_t size;
419 : :
420 : : /* Space for user-defined tranches */
421 : 1244 : ShmemRequestStruct(.name = "LWLock tranches",
422 : : .size = sizeof(LWLockTrancheShmemData),
423 : : .ptr = (void **) &LWLockTranches,
424 : : );
425 : :
426 : : /* Space for the LWLock array */
29 heikki.linnakangas@i 427 [ + - ]:CBC 1244 : if (!IsUnderPostmaster)
428 : : {
29 heikki.linnakangas@i 429 :GNC 1244 : num_main_array_locks = NUM_FIXED_LWLOCKS + NumLWLocksForNamedTranches();
430 : 1244 : size = num_main_array_locks * sizeof(LWLockPadded);
431 : : }
432 : : else
29 heikki.linnakangas@i 433 :UNC 0 : size = SHMEM_ATTACH_UNKNOWN_SIZE;
434 : :
29 heikki.linnakangas@i 435 :GNC 1244 : ShmemRequestStruct(.name = "Main LWLock array",
436 : : .size = size,
437 : : .ptr = (void **) &MainLWLockArray,
438 : : );
3736 rhaas@postgresql.org 439 :CBC 1244 : }
440 : :
441 : : /*
442 : : * Initialize shmem space for user-defined tranches and the main LWLock array.
443 : : */
444 : : static void
29 heikki.linnakangas@i 445 :GNC 1241 : LWLockShmemInit(void *arg)
446 : : {
447 : : int pos;
448 : :
449 : : /* Initialize the dynamic-allocation counter for tranches */
450 : 1241 : LWLockTranches->num_user_defined = 0;
451 : :
452 : 1241 : SpinLockInit(&LWLockTranches->lock);
453 : :
454 : : /*
455 : : * Allocate and initialize all LWLocks in the main array. It includes all
456 : : * LWLocks for built-in tranches and those requested with
457 : : * RequestNamedLWLockTranche().
458 : : */
459 : 1241 : pos = 0;
460 : :
461 : : /* Initialize all individual LWLocks in main array */
40 462 [ + + ]: 73219 : for (int id = 0; id < NUM_INDIVIDUAL_LWLOCKS; id++)
463 : 71978 : LWLockInitialize(&MainLWLockArray[pos++].lock, id);
464 : :
465 : : /* Initialize buffer mapping LWLocks in main array */
466 [ - + ]: 1241 : Assert(pos == BUFFER_MAPPING_LWLOCK_OFFSET);
467 [ + + ]: 160089 : for (int i = 0; i < NUM_BUFFER_PARTITIONS; i++)
468 : 158848 : LWLockInitialize(&MainLWLockArray[pos++].lock, LWTRANCHE_BUFFER_MAPPING);
469 : :
470 : : /* Initialize lmgrs' LWLocks in main array */
471 [ - + ]: 1241 : Assert(pos == LOCK_MANAGER_LWLOCK_OFFSET);
472 [ + + ]: 21097 : for (int i = 0; i < NUM_LOCK_PARTITIONS; i++)
473 : 19856 : LWLockInitialize(&MainLWLockArray[pos++].lock, LWTRANCHE_LOCK_MANAGER);
474 : :
475 : : /* Initialize predicate lmgrs' LWLocks in main array */
476 [ - + ]: 1241 : Assert(pos == PREDICATELOCK_MANAGER_LWLOCK_OFFSET);
477 [ + + ]: 21097 : for (int i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
478 : 19856 : LWLockInitialize(&MainLWLockArray[pos++].lock, LWTRANCHE_PREDICATE_LOCK_MANAGER);
479 : :
480 : : /*
481 : : * Copy the info about any user-defined tranches into shared memory (so
482 : : * that other processes can see it), and initialize the requested LWLocks.
483 : : */
484 [ - + ]: 1241 : Assert(pos == NUM_FIXED_LWLOCKS);
485 [ + + + + : 2484 : foreach_ptr(NamedLWLockTrancheRequest, request, NamedLWLockTrancheRequests)
+ + ]
486 : : {
487 : 2 : int idx = (LWLockTranches->num_user_defined++);
488 : :
489 : 2 : strlcpy(LWLockTranches->user_defined[idx].name,
490 : 2 : request->tranche_name,
491 : : NAMEDATALEN);
492 : 2 : LWLockTranches->user_defined[idx].main_array_idx = pos;
493 : :
494 [ + + ]: 13 : for (int i = 0; i < request->num_lwlocks; i++)
495 : 11 : LWLockInitialize(&MainLWLockArray[pos++].lock, LWTRANCHE_FIRST_USER_DEFINED + idx);
496 : : }
497 : :
498 : : /* Cross-check that we agree on the total size with LWLockShmemRequest() */
29 499 [ - + ]: 1241 : Assert(pos == num_main_array_locks);
3736 rhaas@postgresql.org 500 :CBC 1241 : }
501 : :
502 : : /*
503 : : * InitLWLockAccess - initialize backend-local state needed to hold LWLocks
504 : : */
505 : : void
4327 heikki.linnakangas@i 506 : 22970 : InitLWLockAccess(void)
507 : : {
508 : : #ifdef LWLOCK_STATS
509 : : init_lwlock_stats();
510 : : #endif
511 : 22970 : }
512 : :
513 : : /*
514 : : * GetNamedLWLockTranche - returns the base address of LWLock from the
515 : : * specified tranche.
516 : : *
517 : : * Caller needs to retrieve the requested number of LWLocks starting from
518 : : * the base lock address returned by this API. This can be used for
519 : : * tranches that are requested by using RequestNamedLWLockTranche() API.
520 : : */
521 : : LWLockPadded *
3743 rhaas@postgresql.org 522 : 4 : GetNamedLWLockTranche(const char *tranche_name)
523 : : {
40 heikki.linnakangas@i 524 :GNC 4 : SpinLockAcquire(&LWLockTranches->lock);
525 : 4 : LocalNumUserDefinedTranches = LWLockTranches->num_user_defined;
526 : 4 : SpinLockRelease(&LWLockTranches->lock);
527 : :
528 : : /*
529 : : * Obtain the position of base address of LWLock belonging to requested
530 : : * tranche_name in MainLWLockArray. LWLocks for user-defined tranches
531 : : * requested with RequestNamedLWLockTranche() are placed in
532 : : * MainLWLockArray after fixed locks.
533 : : */
534 [ + + ]: 13 : for (int i = 0; i < LocalNumUserDefinedTranches; i++)
535 : : {
536 [ + + ]: 12 : if (strcmp(LWLockTranches->user_defined[i].name,
537 : : tranche_name) == 0)
538 : : {
539 : 3 : int lock_pos = LWLockTranches->user_defined[i].main_array_idx;
540 : :
541 : : /*
542 : : * GetNamedLWLockTranche() should only be used for locks requested
543 : : * with RequestNamedLWLockTranche(), not those allocated with
544 : : * LWLockNewTrancheId().
545 : : */
546 [ + + ]: 3 : if (lock_pos == -1)
547 [ + - ]: 1 : elog(ERROR, "requested tranche was not registered with RequestNamedLWLockTranche()");
548 : 2 : return &MainLWLockArray[lock_pos];
549 : : }
550 : : }
551 : :
2182 tgl@sss.pgh.pa.us 552 [ + - ]:GBC 1 : elog(ERROR, "requested tranche is not registered");
553 : :
554 : : /* just to keep compiler quiet */
555 : : return NULL;
556 : : }
557 : :
558 : : /*
559 : : * Allocate a new tranche ID with the provided name.
560 : : */
561 : : int
244 nathan@postgresql.or 562 :GNC 305 : LWLockNewTrancheId(const char *name)
563 : : {
564 : : int idx;
565 : :
566 [ + + ]: 305 : if (!name)
567 [ + - ]: 1 : ereport(ERROR,
568 : : (errcode(ERRCODE_INVALID_NAME),
569 : : errmsg("tranche name cannot be NULL")));
570 : :
571 [ + + ]: 304 : if (strlen(name) >= NAMEDATALEN)
572 [ + - ]: 1 : ereport(ERROR,
573 : : (errcode(ERRCODE_NAME_TOO_LONG),
574 : : errmsg("tranche name too long"),
575 : : errdetail("LWLock tranche names must be no longer than %d bytes.",
576 : : NAMEDATALEN - 1)));
577 : :
578 : : /* The counter and the tranche names are protected by the spinlock */
40 heikki.linnakangas@i 579 : 303 : SpinLockAcquire(&LWLockTranches->lock);
580 : :
581 [ + + ]: 303 : if (LWLockTranches->num_user_defined >= MAX_USER_DEFINED_TRANCHES)
582 : : {
583 : 1 : SpinLockRelease(&LWLockTranches->lock);
244 nathan@postgresql.or 584 [ + - ]: 1 : ereport(ERROR,
585 : : (errmsg("maximum number of tranches already registered"),
586 : : errdetail("No more than %d tranches may be registered.",
587 : : MAX_USER_DEFINED_TRANCHES)));
588 : : }
589 : :
590 : : /* Allocate an entry in the user_defined array */
40 heikki.linnakangas@i 591 : 302 : idx = (LWLockTranches->num_user_defined)++;
592 : :
593 : : /* update our local copy while we're at it */
594 : 302 : LocalNumUserDefinedTranches = LWLockTranches->num_user_defined;
595 : :
596 : : /* Initialize it */
597 : 302 : strlcpy(LWLockTranches->user_defined[idx].name, name, NAMEDATALEN);
598 : :
599 : : /* the locks are not in the main array */
600 : 302 : LWLockTranches->user_defined[idx].main_array_idx = -1;
601 : :
602 : 302 : SpinLockRelease(&LWLockTranches->lock);
603 : :
604 : 302 : return LWTRANCHE_FIRST_USER_DEFINED + idx;
605 : : }
606 : :
607 : : /*
608 : : * RequestNamedLWLockTranche
609 : : * Request that extra LWLocks be allocated during postmaster
610 : : * startup.
611 : : *
612 : : * This may only be called via the shmem_request_hook of a library that is
613 : : * loaded into the postmaster via shared_preload_libraries. Calls from
614 : : * elsewhere will fail.
615 : : *
616 : : * The tranche name will be user-visible as a wait event name, so try to
617 : : * use a name that fits the style for those.
618 : : */
619 : : void
3743 rhaas@postgresql.org 620 :CBC 2 : RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
621 : : {
622 : : NamedLWLockTrancheRequest *request;
623 : : MemoryContext oldcontext;
624 : :
1453 625 [ - + ]: 2 : if (!process_shmem_requests_in_progress)
1453 rhaas@postgresql.org 626 [ # # ]:UBC 0 : elog(FATAL, "cannot request additional LWLocks outside shmem_request_hook");
627 : :
244 nathan@postgresql.or 628 [ - + ]:GNC 2 : if (!tranche_name)
244 nathan@postgresql.or 629 [ # # ]:UNC 0 : ereport(ERROR,
630 : : (errcode(ERRCODE_INVALID_NAME),
631 : : errmsg("tranche name cannot be NULL")));
632 : :
244 nathan@postgresql.or 633 [ - + ]:GNC 2 : if (strlen(tranche_name) >= NAMEDATALEN)
244 nathan@postgresql.or 634 [ # # ]:UNC 0 : ereport(ERROR,
635 : : (errcode(ERRCODE_NAME_TOO_LONG),
636 : : errmsg("tranche name too long"),
637 : : errdetail("LWLock tranche names must be no longer than %d bytes.",
638 : : NAMEDATALEN - 1)));
639 : :
40 heikki.linnakangas@i 640 [ - + ]:GNC 2 : if (list_length(NamedLWLockTrancheRequests) >= MAX_USER_DEFINED_TRANCHES)
40 heikki.linnakangas@i 641 [ # # ]:UNC 0 : ereport(ERROR,
642 : : (errmsg("maximum number of tranches already registered"),
643 : : errdetail("No more than %d tranches may be registered.",
644 : : MAX_USER_DEFINED_TRANCHES)));
645 : :
646 : : /* Check that the name isn't already in use */
30 heikki.linnakangas@i 647 [ + + + + :GNC 5 : foreach_ptr(NamedLWLockTrancheRequest, existing, NamedLWLockTrancheRequests)
+ + ]
648 : : {
649 [ - + ]: 1 : if (strcmp(existing->tranche_name, tranche_name) == 0)
30 heikki.linnakangas@i 650 [ # # ]:UNC 0 : elog(ERROR, "requested tranche \"%s\" is already registered", tranche_name);
651 : : }
652 : :
38 heikki.linnakangas@i 653 [ + - ]:GNC 2 : if (IsPostmasterEnvironment)
654 : 2 : oldcontext = MemoryContextSwitchTo(PostmasterContext);
655 : : else
38 heikki.linnakangas@i 656 :UNC 0 : oldcontext = MemoryContextSwitchTo(TopMemoryContext);
657 : :
38 heikki.linnakangas@i 658 :GNC 2 : request = palloc0(sizeof(NamedLWLockTrancheRequest));
2182 tgl@sss.pgh.pa.us 659 :CBC 2 : strlcpy(request->tranche_name, tranche_name, NAMEDATALEN);
3743 rhaas@postgresql.org 660 : 2 : request->num_lwlocks = num_lwlocks;
40 heikki.linnakangas@i 661 :GNC 2 : NamedLWLockTrancheRequests = lappend(NamedLWLockTrancheRequests, request);
662 : :
38 663 : 2 : MemoryContextSwitchTo(oldcontext);
3743 rhaas@postgresql.org 664 :CBC 2 : }
665 : :
666 : : /*
667 : : * LWLockInitialize - initialize a new lwlock; it's initially unlocked
668 : : */
669 : : void
4481 670 : 2575279 : LWLockInitialize(LWLock *lock, int tranche_id)
671 : : {
672 : : /* verify the tranche_id is valid */
244 nathan@postgresql.or 673 :GNC 2575279 : (void) GetLWTrancheName(tranche_id);
674 : :
112 andres@anarazel.de 675 : 2575278 : pg_atomic_init_u32(&lock->state, 0);
676 : : #ifdef LOCK_DEBUG
677 : : pg_atomic_init_u32(&lock->nwaiters, 0);
678 : : #endif
4481 rhaas@postgresql.org 679 :CBC 2575278 : lock->tranche = tranche_id;
3550 680 : 2575278 : proclist_init(&lock->waiters);
4481 681 : 2575278 : }
682 : :
683 : : /*
684 : : * Report start of wait event for light-weight locks.
685 : : *
686 : : * This function will be used by all the light-weight lock calls which
687 : : * needs to wait to acquire the lock. This function distinguishes wait
688 : : * event based on tranche and lock id.
689 : : */
690 : : static inline void
3708 691 : 260274 : LWLockReportWaitStart(LWLock *lock)
692 : : {
3427 693 : 260274 : pgstat_report_wait_start(PG_WAIT_LWLOCK | lock->tranche);
3708 694 : 260274 : }
695 : :
696 : : /*
697 : : * Report end of wait event for light-weight locks.
698 : : */
699 : : static inline void
3691 andres@anarazel.de 700 : 260274 : LWLockReportWaitEnd(void)
701 : : {
3708 rhaas@postgresql.org 702 : 260274 : pgstat_report_wait_end();
703 : 260274 : }
704 : :
705 : : /*
706 : : * Return the name of an LWLock tranche.
707 : : */
708 : : static const char *
2182 tgl@sss.pgh.pa.us 709 : 2575300 : GetLWTrancheName(uint16 trancheId)
710 : : {
711 : : int idx;
712 : :
713 : : /* Built-in tranche or individual LWLock? */
714 [ + + ]: 2575300 : if (trancheId < LWTRANCHE_FIRST_USER_DEFINED)
832 alvherre@alvh.no-ip. 715 : 2572888 : return BuiltinTrancheNames[trancheId];
716 : :
717 : : /*
718 : : * It's an extension tranche, so look in LWLockTranches->user_defined.
719 : : */
40 heikki.linnakangas@i 720 :GNC 2412 : idx = trancheId - LWTRANCHE_FIRST_USER_DEFINED;
721 : :
722 : : /*
723 : : * We only ever add new entries to LWLockTranches->user_defined, so most
724 : : * lookups can avoid taking the spinlock as long as the backend-local
725 : : * counter (LocalNumUserDefinedTranches) is greater than the requested
726 : : * tranche ID. Else, we need to first update the backend-local counter
727 : : * with the spinlock held before attempting the lookup again. In
728 : : * practice, the latter case is probably rare.
729 : : */
730 [ + + ]: 2412 : if (idx >= LocalNumUserDefinedTranches)
731 : : {
732 : 3 : SpinLockAcquire(&LWLockTranches->lock);
733 : 3 : LocalNumUserDefinedTranches = LWLockTranches->num_user_defined;
734 : 3 : SpinLockRelease(&LWLockTranches->lock);
735 : :
736 [ + + ]: 3 : if (idx >= LocalNumUserDefinedTranches)
244 nathan@postgresql.or 737 [ + - ]: 1 : elog(ERROR, "tranche %d is not registered", trancheId);
738 : : }
739 : :
40 heikki.linnakangas@i 740 : 2411 : return LWLockTranches->user_defined[idx].name;
741 : : }
742 : :
743 : : /*
744 : : * Return an identifier for an LWLock based on the wait class and event.
745 : : */
746 : : const char *
2182 tgl@sss.pgh.pa.us 747 :CBC 21 : GetLWLockIdentifier(uint32 classId, uint16 eventId)
748 : : {
749 [ - + ]: 21 : Assert(classId == PG_WAIT_LWLOCK);
750 : : /* The event IDs are just tranche numbers. */
751 : 21 : return GetLWTrancheName(eventId);
752 : : }
753 : :
754 : : /*
755 : : * Internal function that tries to atomically acquire the lwlock in the passed
756 : : * in mode.
757 : : *
758 : : * This function will not block waiting for a lock to become free - that's the
759 : : * caller's job.
760 : : *
761 : : * Returns true if the lock isn't free and we need to wait.
762 : : */
763 : : static bool
4000 bruce@momjian.us 764 : 222310990 : LWLockAttemptLock(LWLock *lock, LWLockMode mode)
765 : : {
766 : : uint32 old_state;
767 : :
1285 peter@eisentraut.org 768 [ + + - + ]: 222310990 : Assert(mode == LW_EXCLUSIVE || mode == LW_SHARED);
769 : :
770 : : /*
771 : : * Read once outside the loop, later iterations will get the newer value
772 : : * via compare & exchange.
773 : : */
3931 andres@anarazel.de 774 : 222310990 : old_state = pg_atomic_read_u32(&lock->state);
775 : :
776 : : /* loop until we've determined whether we could acquire the lock or not */
777 : : while (true)
4149 778 : 13414 : {
779 : : uint32 desired_state;
780 : : bool lock_free;
781 : :
3931 782 : 222324404 : desired_state = old_state;
783 : :
4149 784 [ + + ]: 222324404 : if (mode == LW_EXCLUSIVE)
785 : : {
3931 786 : 126914587 : lock_free = (old_state & LW_LOCK_MASK) == 0;
4149 787 [ + + ]: 126914587 : if (lock_free)
788 : 126684904 : desired_state += LW_VAL_EXCLUSIVE;
789 : : }
790 : : else
791 : : {
3931 792 : 95409817 : lock_free = (old_state & LW_VAL_EXCLUSIVE) == 0;
4149 793 [ + + ]: 95409817 : if (lock_free)
794 : 95408789 : desired_state += LW_VAL_SHARED;
795 : : }
796 : :
797 : : /*
798 : : * Attempt to swap in the state we are expecting. If we didn't see
799 : : * lock to be free, that's just the old value. If we saw it as free,
800 : : * we'll attempt to mark it acquired. The reason that we always swap
801 : : * in the value is that this doubles as a memory barrier. We could try
802 : : * to be smarter and only swap in values if we saw the lock as free,
803 : : * but benchmark haven't shown it as beneficial so far.
804 : : *
805 : : * Retry if the value changed since we last looked at it.
806 : : */
807 [ + + ]: 222324404 : if (pg_atomic_compare_exchange_u32(&lock->state,
808 : : &old_state, desired_state))
809 : : {
810 [ + + ]: 222310990 : if (lock_free)
811 : : {
812 : : /* Great! Got the lock. */
813 : : #ifdef LOCK_DEBUG
814 : : if (mode == LW_EXCLUSIVE)
815 : : lock->owner = MyProc;
816 : : #endif
817 : 222083727 : return false;
818 : : }
819 : : else
3375 heikki.linnakangas@i 820 : 227263 : return true; /* somebody else has the lock */
821 : : }
822 : : }
823 : : pg_unreachable();
824 : : }
825 : :
826 : : /*
827 : : * Lock the LWLock's wait list against concurrent activity.
828 : : *
829 : : * NB: even though the wait list is locked, non-conflicting lock operations
830 : : * may still happen concurrently.
831 : : *
832 : : * Time spent holding mutex should be short!
833 : : */
834 : : static void
3677 andres@anarazel.de 835 : 3170217 : LWLockWaitListLock(LWLock *lock)
836 : : {
837 : : uint32 old_state;
838 : : #ifdef LWLOCK_STATS
839 : : lwlock_stats *lwstats;
840 : : uint32 delays = 0;
841 : :
842 : : lwstats = get_lwlock_stats_entry(lock);
843 : : #endif
844 : :
845 : : while (true)
846 : : {
847 : : /*
848 : : * Always try once to acquire the lock directly, without setting up
849 : : * the spin-delay infrastructure. The work necessary for that shows up
850 : : * in profiles and is rarely necessary.
851 : : */
852 : 3177710 : old_state = pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_LOCKED);
137 andres@anarazel.de 853 [ + + ]:GNC 3177710 : if (likely(!(old_state & LW_FLAG_LOCKED)))
3677 andres@anarazel.de 854 :CBC 3170217 : break; /* got lock */
855 : :
856 : : /* and then spin without atomic operations until lock is released */
857 : : {
858 : : SpinDelayStatus delayStatus;
859 : :
3673 860 : 7493 : init_local_spin_delay(&delayStatus);
861 : :
3677 862 [ + + ]: 73838 : while (old_state & LW_FLAG_LOCKED)
863 : : {
864 : 66345 : perform_spin_delay(&delayStatus);
865 : 66345 : old_state = pg_atomic_read_u32(&lock->state);
866 : : }
867 : : #ifdef LWLOCK_STATS
868 : : delays += delayStatus.delays;
869 : : #endif
870 : 7493 : finish_spin_delay(&delayStatus);
871 : : }
872 : :
873 : : /*
874 : : * Retry. The lock might obviously already be re-acquired by the time
875 : : * we're attempting to get it again.
876 : : */
877 : : }
878 : :
879 : : #ifdef LWLOCK_STATS
880 : : lwstats->spin_delay_count += delays;
881 : : #endif
882 : 3170217 : }
883 : :
884 : : /*
885 : : * Unlock the LWLock's wait list.
886 : : *
887 : : * Note that it can be more efficient to manipulate flags and release the
888 : : * locks in a single atomic operation.
889 : : */
890 : : static void
891 : 3003694 : LWLockWaitListUnlock(LWLock *lock)
892 : : {
893 : : uint32 old_state PG_USED_FOR_ASSERTS_ONLY;
894 : :
895 : 3003694 : old_state = pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_LOCKED);
896 : :
897 [ - + ]: 3003694 : Assert(old_state & LW_FLAG_LOCKED);
898 : 3003694 : }
899 : :
900 : : /*
901 : : * Wakeup all the lockers that currently have a chance to acquire the lock.
902 : : */
903 : : static void
4149 904 : 166523 : LWLockWakeup(LWLock *lock)
905 : : {
111 andres@anarazel.de 906 :GNC 166523 : bool new_wake_in_progress = false;
4149 andres@anarazel.de 907 :CBC 166523 : bool wokeup_somebody = false;
908 : : proclist_head wakeup;
909 : : proclist_mutable_iter iter;
910 : :
3550 rhaas@postgresql.org 911 : 166523 : proclist_init(&wakeup);
912 : :
913 : : /* lock wait list while collecting backends to wake up */
3677 andres@anarazel.de 914 : 166523 : LWLockWaitListLock(lock);
915 : :
3550 rhaas@postgresql.org 916 [ + + + + : 324877 : proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
+ + ]
917 : : {
918 : 263955 : PGPROC *waiter = GetPGProcByNumber(iter.cur);
919 : :
4149 andres@anarazel.de 920 [ + + + + ]: 263955 : if (wokeup_somebody && waiter->lwWaitMode == LW_EXCLUSIVE)
921 : 17 : continue;
922 : :
3550 rhaas@postgresql.org 923 : 263938 : proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
924 : 263938 : proclist_push_tail(&wakeup, iter.cur, lwWaitLink);
925 : :
4149 andres@anarazel.de 926 [ + + ]: 263938 : if (waiter->lwWaitMode != LW_WAIT_UNTIL_FREE)
927 : : {
928 : : /*
929 : : * Prevent additional wakeups until retryer gets to run. Backends
930 : : * that are just waiting for the lock to become free don't retry
931 : : * automatically.
932 : : */
111 andres@anarazel.de 933 :GNC 105994 : new_wake_in_progress = true;
934 : :
935 : : /*
936 : : * Don't wakeup (further) exclusive locks.
937 : : */
4149 andres@anarazel.de 938 :CBC 105994 : wokeup_somebody = true;
939 : : }
940 : :
941 : : /*
942 : : * Signal that the process isn't on the wait list anymore. This allows
943 : : * LWLockDequeueSelf() to remove itself of the waitlist with a
944 : : * proclist_delete(), rather than having to check if it has been
945 : : * removed from the list.
946 : : */
1262 947 [ - + ]: 263938 : Assert(waiter->lwWaiting == LW_WS_WAITING);
948 : 263938 : waiter->lwWaiting = LW_WS_PENDING_WAKEUP;
949 : :
950 : : /*
951 : : * Once we've woken up an exclusive lock, there's no point in waking
952 : : * up anybody else.
953 : : */
4000 bruce@momjian.us 954 [ + + ]: 263938 : if (waiter->lwWaitMode == LW_EXCLUSIVE)
4149 andres@anarazel.de 955 : 105601 : break;
956 : : }
957 : :
3550 rhaas@postgresql.org 958 [ + + - + ]: 166523 : Assert(proclist_is_empty(&wakeup) || pg_atomic_read_u32(&lock->state) & LW_FLAG_HAS_WAITERS);
959 : :
960 : : /* unset required flags, and release lock, in one fell swoop */
961 : : {
962 : : uint32 old_state;
963 : : uint32 desired_state;
964 : :
3677 andres@anarazel.de 965 : 166523 : old_state = pg_atomic_read_u32(&lock->state);
966 : : while (true)
967 : : {
968 : 166801 : desired_state = old_state;
969 : :
970 : : /* compute desired flags */
971 : :
111 andres@anarazel.de 972 [ + + ]:GNC 166801 : if (new_wake_in_progress)
112 973 : 105992 : desired_state |= LW_FLAG_WAKE_IN_PROGRESS;
974 : : else
975 : 60809 : desired_state &= ~LW_FLAG_WAKE_IN_PROGRESS;
976 : :
162 andres@anarazel.de 977 [ + + ]:CBC 166801 : if (proclist_is_empty(&lock->waiters))
3677 978 : 136969 : desired_state &= ~LW_FLAG_HAS_WAITERS;
979 : :
980 : 166801 : desired_state &= ~LW_FLAG_LOCKED; /* release lock */
981 : :
982 [ + + ]: 166801 : if (pg_atomic_compare_exchange_u32(&lock->state, &old_state,
983 : : desired_state))
984 : 166523 : break;
985 : : }
986 : : }
987 : :
988 : : /* Awaken any waiters I removed from the queue. */
3550 rhaas@postgresql.org 989 [ + + + + : 430461 : proclist_foreach_modify(iter, &wakeup, lwWaitLink)
+ + ]
990 : : {
991 : 263938 : PGPROC *waiter = GetPGProcByNumber(iter.cur);
992 : :
993 : : LOG_LWDEBUG("LWLockRelease", lock, "release waiter");
994 : 263938 : proclist_delete(&wakeup, iter.cur, lwWaitLink);
995 : :
996 : : /*
997 : : * Guarantee that lwWaiting being unset only becomes visible once the
998 : : * unlink from the link has completed. Otherwise the target backend
999 : : * could be woken up for other reason and enqueue for a new lock - if
1000 : : * that happens before the list unlink happens, the list would end up
1001 : : * being corrupted.
1002 : : *
1003 : : * The barrier pairs with the LWLockWaitListLock() when enqueuing for
1004 : : * another lock.
1005 : : */
4149 andres@anarazel.de 1006 : 263938 : pg_write_barrier();
1262 1007 : 263938 : waiter->lwWaiting = LW_WS_NOT_WAITING;
3431 tgl@sss.pgh.pa.us 1008 : 263938 : PGSemaphoreUnlock(waiter->sem);
1009 : : }
4149 andres@anarazel.de 1010 : 166523 : }
1011 : :
1012 : : /*
1013 : : * Add ourselves to the end of the queue.
1014 : : *
1015 : : * NB: Mode can be LW_WAIT_UNTIL_FREE here!
1016 : : */
1017 : : static void
1018 : 275008 : LWLockQueueSelf(LWLock *lock, LWLockMode mode)
1019 : : {
1020 : : /*
1021 : : * If we don't have a PGPROC structure, there's no way to wait. This
1022 : : * should never occur, since MyProc should only be null during shared
1023 : : * memory initialization.
1024 : : */
1025 [ - + ]: 275008 : if (MyProc == NULL)
4149 andres@anarazel.de 1026 [ # # ]:UBC 0 : elog(PANIC, "cannot wait without a PGPROC structure");
1027 : :
1262 andres@anarazel.de 1028 [ - + ]:CBC 275008 : if (MyProc->lwWaiting != LW_WS_NOT_WAITING)
4149 andres@anarazel.de 1029 [ # # ]:UBC 0 : elog(PANIC, "queueing for lock while waiting on another one");
1030 : :
3677 andres@anarazel.de 1031 :CBC 275008 : LWLockWaitListLock(lock);
1032 : :
1033 : : /* setting the flag is protected by the spinlock */
4149 1034 : 275008 : pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_HAS_WAITERS);
1035 : :
1262 1036 : 275008 : MyProc->lwWaiting = LW_WS_WAITING;
4149 1037 : 275008 : MyProc->lwWaitMode = mode;
1038 : :
1039 : : /* LW_WAIT_UNTIL_FREE waiters are always at the front of the queue */
1040 [ + + ]: 275008 : if (mode == LW_WAIT_UNTIL_FREE)
803 heikki.linnakangas@i 1041 : 159122 : proclist_push_head(&lock->waiters, MyProcNumber, lwWaitLink);
1042 : : else
1043 : 115886 : proclist_push_tail(&lock->waiters, MyProcNumber, lwWaitLink);
1044 : :
1045 : : /* Can release the mutex now */
3677 andres@anarazel.de 1046 : 275008 : LWLockWaitListUnlock(lock);
1047 : :
1048 : : #ifdef LOCK_DEBUG
1049 : : pg_atomic_fetch_add_u32(&lock->nwaiters, 1);
1050 : : #endif
4149 1051 : 275008 : }
1052 : :
1053 : : /*
1054 : : * Remove ourselves from the waitlist.
1055 : : *
1056 : : * This is used if we queued ourselves because we thought we needed to sleep
1057 : : * but, after further checking, we discovered that we don't actually need to
1058 : : * do so.
1059 : : */
1060 : : static void
1061 : 14734 : LWLockDequeueSelf(LWLock *lock)
1062 : : {
1063 : : bool on_waitlist;
1064 : :
1065 : : #ifdef LWLOCK_STATS
1066 : : lwlock_stats *lwstats;
1067 : :
1068 : : lwstats = get_lwlock_stats_entry(lock);
1069 : :
1070 : : lwstats->dequeue_self_count++;
1071 : : #endif
1072 : :
3677 1073 : 14734 : LWLockWaitListLock(lock);
1074 : :
1075 : : /*
1076 : : * Remove ourselves from the waitlist, unless we've already been removed.
1077 : : * The removal happens with the wait list lock held, so there's no race in
1078 : : * this check.
1079 : : */
1262 1080 : 14734 : on_waitlist = MyProc->lwWaiting == LW_WS_WAITING;
1081 [ + + ]: 14734 : if (on_waitlist)
803 heikki.linnakangas@i 1082 : 10624 : proclist_delete(&lock->waiters, MyProcNumber, lwWaitLink);
1083 : :
3550 rhaas@postgresql.org 1084 [ + + ]: 14734 : if (proclist_is_empty(&lock->waiters) &&
4149 andres@anarazel.de 1085 [ + + ]: 14136 : (pg_atomic_read_u32(&lock->state) & LW_FLAG_HAS_WAITERS) != 0)
1086 : : {
1087 : 10235 : pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_HAS_WAITERS);
1088 : : }
1089 : :
1090 : : /* XXX: combine with fetch_and above? */
3677 1091 : 14734 : LWLockWaitListUnlock(lock);
1092 : :
1093 : : /* clear waiting state again, nice for debugging */
1262 1094 [ + + ]: 14734 : if (on_waitlist)
1095 : 10624 : MyProc->lwWaiting = LW_WS_NOT_WAITING;
1096 : : else
1097 : : {
4000 bruce@momjian.us 1098 : 4110 : int extraWaits = 0;
1099 : :
1100 : : /*
1101 : : * Somebody else dequeued us and has or will wake us up. Deal with the
1102 : : * superfluous absorption of a wakeup.
1103 : : */
1104 : :
1105 : : /*
1106 : : * Clear LW_FLAG_WAKE_IN_PROGRESS if somebody woke us before we
1107 : : * removed ourselves - they'll have set it.
1108 : : */
112 andres@anarazel.de 1109 :GNC 4110 : pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_WAKE_IN_PROGRESS);
1110 : :
1111 : : /*
1112 : : * Now wait for the scheduled wakeup, otherwise our ->lwWaiting would
1113 : : * get reset at some inconvenient point later. Most of the time this
1114 : : * will immediately return.
1115 : : */
1116 : : for (;;)
1117 : : {
3431 tgl@sss.pgh.pa.us 1118 :CBC 4110 : PGSemaphoreLock(MyProc->sem);
1262 andres@anarazel.de 1119 [ + - ]: 4110 : if (MyProc->lwWaiting == LW_WS_NOT_WAITING)
4149 1120 : 4110 : break;
4149 andres@anarazel.de 1121 :UBC 0 : extraWaits++;
1122 : : }
1123 : :
1124 : : /*
1125 : : * Fix the process wait semaphore's count for any absorbed wakeups.
1126 : : */
4149 andres@anarazel.de 1127 [ - + ]:CBC 4110 : while (extraWaits-- > 0)
3431 tgl@sss.pgh.pa.us 1128 :UBC 0 : PGSemaphoreUnlock(MyProc->sem);
1129 : : }
1130 : :
1131 : : #ifdef LOCK_DEBUG
1132 : : {
1133 : : /* not waiting anymore */
1134 : : uint32 nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);
1135 : :
1136 : : Assert(nwaiters < MAX_BACKENDS);
1137 : : }
1138 : : #endif
4149 andres@anarazel.de 1139 :CBC 14734 : }
1140 : :
1141 : : /*
1142 : : * LWLockAcquire - acquire a lightweight lock in the specified mode
1143 : : *
1144 : : * If the lock is not available, sleep until it is. Returns true if the lock
1145 : : * was available immediately, false if we had to sleep.
1146 : : *
1147 : : * Side effect: cancel/die interrupts are held off until lock release.
1148 : : */
1149 : : bool
3931 1150 : 220407724 : LWLockAcquire(LWLock *lock, LWLockMode mode)
1151 : : {
8729 JanWieck@Yahoo.com 1152 : 220407724 : PGPROC *proc = MyProc;
4428 heikki.linnakangas@i 1153 : 220407724 : bool result = true;
8884 tgl@sss.pgh.pa.us 1154 : 220407724 : int extraWaits = 0;
1155 : : #ifdef LWLOCK_STATS
1156 : : lwlock_stats *lwstats;
1157 : :
1158 : : lwstats = get_lwlock_stats_entry(lock);
1159 : : #endif
1160 : :
1285 peter@eisentraut.org 1161 [ + + - + ]: 220407724 : Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);
1162 : :
1163 : : PRINT_LWDEBUG("LWLockAcquire", lock, mode);
1164 : :
1165 : : #ifdef LWLOCK_STATS
1166 : : /* Count lock acquisition attempts */
1167 : : if (mode == LW_EXCLUSIVE)
1168 : : lwstats->ex_acquire_count++;
1169 : : else
1170 : : lwstats->sh_acquire_count++;
1171 : : #endif /* LWLOCK_STATS */
1172 : :
1173 : : /*
1174 : : * We can't wait if we haven't got a PGPROC. This should only occur
1175 : : * during bootstrap or shared memory initialization. Put an Assert here
1176 : : * to catch unsafe coding practices.
1177 : : */
8623 tgl@sss.pgh.pa.us 1178 [ + + - + ]: 220407724 : Assert(!(proc == NULL && IsUnderPostmaster));
1179 : :
1180 : : /* Ensure we will have room to remember the lock */
7697 1181 [ - + ]: 220407724 : if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
7697 tgl@sss.pgh.pa.us 1182 [ # # ]:UBC 0 : elog(ERROR, "too many LWLocks taken");
1183 : :
1184 : : /*
1185 : : * Lock out cancel/die interrupts until we exit the code section protected
1186 : : * by the LWLock. This ensures that interrupts will not interfere with
1187 : : * manipulations of data structures in shared memory.
1188 : : */
8984 tgl@sss.pgh.pa.us 1189 :CBC 220407724 : HOLD_INTERRUPTS();
1190 : :
1191 : : /*
1192 : : * Loop here to try to acquire lock after each time we are signaled by
1193 : : * LWLockRelease.
1194 : : *
1195 : : * NOTE: it might seem better to have LWLockRelease actually grant us the
1196 : : * lock, rather than retrying and possibly having to go back to sleep. But
1197 : : * in practice that is no good because it means a process swap for every
1198 : : * lock acquisition when two or more processes are contending for the same
1199 : : * lock. Since LWLocks are normally used to protect not-very-long
1200 : : * sections of computation, a process needs to be able to acquire and
1201 : : * release the same lock many times during a single CPU time slice, even
1202 : : * in the presence of contention. The efficiency of being able to do that
1203 : : * outweighs the inefficiency of sometimes wasting a process dispatch
1204 : : * cycle because the lock is not free when a released waiter finally gets
1205 : : * to run. See pgsql-hackers archives for 29-Dec-01.
1206 : : */
1207 : : for (;;)
8893 bruce@momjian.us 1208 : 102019 : {
1209 : : bool mustwait;
1210 : :
1211 : : /*
1212 : : * Try to grab the lock the first time, we're not in the waitqueue
1213 : : * yet/anymore.
1214 : : */
4149 andres@anarazel.de 1215 : 220509743 : mustwait = LWLockAttemptLock(lock, mode);
1216 : :
8884 tgl@sss.pgh.pa.us 1217 [ + + ]: 220509743 : if (!mustwait)
1218 : : {
1219 : : LOG_LWDEBUG("LWLockAcquire", lock, "immediately acquired lock");
1220 : 220393857 : break; /* got the lock */
1221 : : }
1222 : :
1223 : : /*
1224 : : * Ok, at this point we couldn't grab the lock on the first try. We
1225 : : * cannot simply queue ourselves to the end of the list and wait to be
1226 : : * woken up because by now the lock could long have been released.
1227 : : * Instead add us to the queue and try to grab the lock again. If we
1228 : : * succeed we need to revert the queuing and be happy, otherwise we
1229 : : * recheck the lock. If we still couldn't grab it, we know that the
1230 : : * other locker will see our queue entries when releasing since they
1231 : : * existed before we checked for the lock.
1232 : : */
1233 : :
1234 : : /* add to the queue */
4149 andres@anarazel.de 1235 : 115886 : LWLockQueueSelf(lock, mode);
1236 : :
1237 : : /* we're now guaranteed to be woken up if necessary */
1238 : 115886 : mustwait = LWLockAttemptLock(lock, mode);
1239 : :
1240 : : /* ok, grabbed the lock the second time round, need to undo queueing */
1241 [ + + ]: 115886 : if (!mustwait)
1242 : : {
1243 : : LOG_LWDEBUG("LWLockAcquire", lock, "acquired, undoing queue");
1244 : :
1245 : 13867 : LWLockDequeueSelf(lock);
1246 : 13867 : break;
1247 : : }
1248 : :
1249 : : /*
1250 : : * Wait until awakened.
1251 : : *
1252 : : * It is possible that we get awakened for a reason other than being
1253 : : * signaled by LWLockRelease. If so, loop back and wait again. Once
1254 : : * we've gotten the LWLock, re-increment the sema by the number of
1255 : : * additional signals received.
1256 : : */
1257 : : LOG_LWDEBUG("LWLockAcquire", lock, "waiting");
1258 : :
1259 : : #ifdef LWLOCK_STATS
1260 : : lwstats->block_count++;
1261 : : #endif
1262 : :
3708 rhaas@postgresql.org 1263 : 102019 : LWLockReportWaitStart(lock);
1264 : : if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
1265 : : TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);
1266 : :
1267 : : for (;;)
1268 : : {
3431 tgl@sss.pgh.pa.us 1269 : 102019 : PGSemaphoreLock(proc->sem);
1262 andres@anarazel.de 1270 [ + - ]: 102019 : if (proc->lwWaiting == LW_WS_NOT_WAITING)
8984 tgl@sss.pgh.pa.us 1271 : 102019 : break;
8984 tgl@sss.pgh.pa.us 1272 :UBC 0 : extraWaits++;
1273 : : }
1274 : :
1275 : : /* Retrying, allow LWLockRelease to release waiters again. */
112 andres@anarazel.de 1276 :GNC 102019 : pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_WAKE_IN_PROGRESS);
1277 : :
1278 : : #ifdef LOCK_DEBUG
1279 : : {
1280 : : /* not waiting anymore */
1281 : : uint32 nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);
1282 : :
1283 : : Assert(nwaiters < MAX_BACKENDS);
1284 : : }
1285 : : #endif
1286 : :
1287 : : if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
1288 : : TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
3708 rhaas@postgresql.org 1289 :CBC 102019 : LWLockReportWaitEnd();
1290 : :
1291 : : LOG_LWDEBUG("LWLockAcquire", lock, "awakened");
1292 : :
1293 : : /* Now loop back and try to acquire lock again. */
4428 heikki.linnakangas@i 1294 : 102019 : result = false;
1295 : : }
1296 : :
1297 : : if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_ENABLED())
1298 : : TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), mode);
1299 : :
1300 : : /* Add lock to list of locks held by this backend */
4149 andres@anarazel.de 1301 : 220407724 : held_lwlocks[num_held_lwlocks].lock = lock;
1302 : 220407724 : held_lwlocks[num_held_lwlocks++].mode = mode;
1303 : :
1304 : : /*
1305 : : * Fix the process wait semaphore's count for any absorbed wakeups.
1306 : : */
8884 tgl@sss.pgh.pa.us 1307 [ - + ]: 220407724 : while (extraWaits-- > 0)
3431 tgl@sss.pgh.pa.us 1308 :UBC 0 : PGSemaphoreUnlock(proc->sem);
1309 : :
4428 heikki.linnakangas@i 1310 :CBC 220407724 : return result;
1311 : : }
1312 : :
1313 : : /*
1314 : : * LWLockConditionalAcquire - acquire a lightweight lock in the specified mode
1315 : : *
1316 : : * If the lock is not available, return false with no side-effects.
1317 : : *
1318 : : * If successful, cancel/die interrupts are held off until lock release.
1319 : : */
1320 : : bool
4243 rhaas@postgresql.org 1321 : 1514556 : LWLockConditionalAcquire(LWLock *lock, LWLockMode mode)
1322 : : {
1323 : : bool mustwait;
1324 : :
1285 peter@eisentraut.org 1325 [ + + - + ]: 1514556 : Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);
1326 : :
1327 : : PRINT_LWDEBUG("LWLockConditionalAcquire", lock, mode);
1328 : :
1329 : : /* Ensure we will have room to remember the lock */
7697 tgl@sss.pgh.pa.us 1330 [ - + ]: 1514556 : if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
7697 tgl@sss.pgh.pa.us 1331 [ # # ]:UBC 0 : elog(ERROR, "too many LWLocks taken");
1332 : :
1333 : : /*
1334 : : * Lock out cancel/die interrupts until we exit the code section protected
1335 : : * by the LWLock. This ensures that interrupts will not interfere with
1336 : : * manipulations of data structures in shared memory.
1337 : : */
8984 tgl@sss.pgh.pa.us 1338 :CBC 1514556 : HOLD_INTERRUPTS();
1339 : :
1340 : : /* Check for the lock */
4149 andres@anarazel.de 1341 : 1514556 : mustwait = LWLockAttemptLock(lock, mode);
1342 : :
8984 tgl@sss.pgh.pa.us 1343 [ + + ]: 1514556 : if (mustwait)
1344 : : {
1345 : : /* Failed to get lock, so release interrupt holdoff */
1346 [ - + ]: 2956 : RESUME_INTERRUPTS();
1347 : :
1348 : : LOG_LWDEBUG("LWLockConditionalAcquire", lock, "failed");
1349 : : if (TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL_ENABLED())
1350 : : TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL(T_NAME(lock), mode);
1351 : : }
1352 : : else
1353 : : {
1354 : : /* Add lock to list of locks held by this backend */
4149 andres@anarazel.de 1355 : 1511600 : held_lwlocks[num_held_lwlocks].lock = lock;
1356 : 1511600 : held_lwlocks[num_held_lwlocks++].mode = mode;
1357 : : if (TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_ENABLED())
1358 : : TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE(T_NAME(lock), mode);
1359 : : }
8984 tgl@sss.pgh.pa.us 1360 : 1514556 : return !mustwait;
1361 : : }
1362 : :
1363 : : /*
1364 : : * LWLockAcquireOrWait - Acquire lock, or wait until it's free
1365 : : *
1366 : : * The semantics of this function are a bit funky. If the lock is currently
1367 : : * free, it is acquired in the given mode, and the function returns true. If
1368 : : * the lock isn't immediately free, the function waits until it is released
1369 : : * and returns false, but does not acquire the lock.
1370 : : *
1371 : : * This is currently used for WALWriteLock: when a backend flushes the WAL,
1372 : : * holding WALWriteLock, it can flush the commit records of many other
1373 : : * backends as a side-effect. Those other backends need to wait until the
1374 : : * flush finishes, but don't need to acquire the lock anymore. They can just
1375 : : * wake up, observe that their records have already been flushed, and return.
1376 : : */
1377 : : bool
4243 rhaas@postgresql.org 1378 : 167562 : LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
1379 : : {
5209 heikki.linnakangas@i 1380 : 167562 : PGPROC *proc = MyProc;
1381 : : bool mustwait;
1382 : 167562 : int extraWaits = 0;
1383 : : #ifdef LWLOCK_STATS
1384 : : lwlock_stats *lwstats;
1385 : :
1386 : : lwstats = get_lwlock_stats_entry(lock);
1387 : : #endif
1388 : :
4149 andres@anarazel.de 1389 [ + - - + ]: 167562 : Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);
1390 : :
1391 : : PRINT_LWDEBUG("LWLockAcquireOrWait", lock, mode);
1392 : :
1393 : : /* Ensure we will have room to remember the lock */
5209 heikki.linnakangas@i 1394 [ - + ]: 167562 : if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
5209 heikki.linnakangas@i 1395 [ # # ]:UBC 0 : elog(ERROR, "too many LWLocks taken");
1396 : :
1397 : : /*
1398 : : * Lock out cancel/die interrupts until we exit the code section protected
1399 : : * by the LWLock. This ensures that interrupts will not interfere with
1400 : : * manipulations of data structures in shared memory.
1401 : : */
5209 heikki.linnakangas@i 1402 :CBC 167562 : HOLD_INTERRUPTS();
1403 : :
1404 : : /*
1405 : : * NB: We're using nearly the same twice-in-a-row lock acquisition
1406 : : * protocol as LWLockAcquire(). Check its comments for details.
1407 : : */
4149 andres@anarazel.de 1408 : 167562 : mustwait = LWLockAttemptLock(lock, mode);
1409 : :
5209 heikki.linnakangas@i 1410 [ + + ]: 167562 : if (mustwait)
1411 : : {
4149 andres@anarazel.de 1412 : 3243 : LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);
1413 : :
1414 : 3243 : mustwait = LWLockAttemptLock(lock, mode);
1415 : :
1416 [ + + ]: 3243 : if (mustwait)
1417 : : {
1418 : : /*
1419 : : * Wait until awakened. Like in LWLockAcquire, be prepared for
1420 : : * bogus wakeups.
1421 : : */
1422 : : LOG_LWDEBUG("LWLockAcquireOrWait", lock, "waiting");
1423 : :
1424 : : #ifdef LWLOCK_STATS
1425 : : lwstats->block_count++;
1426 : : #endif
1427 : :
3708 rhaas@postgresql.org 1428 : 3159 : LWLockReportWaitStart(lock);
1429 : : if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
1430 : : TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);
1431 : :
1432 : : for (;;)
1433 : : {
3431 tgl@sss.pgh.pa.us 1434 : 3159 : PGSemaphoreLock(proc->sem);
1262 andres@anarazel.de 1435 [ + - ]: 3159 : if (proc->lwWaiting == LW_WS_NOT_WAITING)
4149 1436 : 3159 : break;
4149 andres@anarazel.de 1437 :UBC 0 : extraWaits++;
1438 : : }
1439 : :
1440 : : #ifdef LOCK_DEBUG
1441 : : {
1442 : : /* not waiting anymore */
1443 : : uint32 nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);
1444 : :
1445 : : Assert(nwaiters < MAX_BACKENDS);
1446 : : }
1447 : : #endif
1448 : : if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
1449 : : TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
3708 rhaas@postgresql.org 1450 :CBC 3159 : LWLockReportWaitEnd();
1451 : :
1452 : : LOG_LWDEBUG("LWLockAcquireOrWait", lock, "awakened");
1453 : : }
1454 : : else
1455 : : {
1456 : : LOG_LWDEBUG("LWLockAcquireOrWait", lock, "acquired, undoing queue");
1457 : :
1458 : : /*
1459 : : * Got lock in the second attempt, undo queueing. We need to treat
1460 : : * this as having successfully acquired the lock, otherwise we'd
1461 : : * not necessarily wake up people we've prevented from acquiring
1462 : : * the lock.
1463 : : */
4149 andres@anarazel.de 1464 : 84 : LWLockDequeueSelf(lock);
1465 : : }
1466 : : }
1467 : :
1468 : : /*
1469 : : * Fix the process wait semaphore's count for any absorbed wakeups.
1470 : : */
5209 heikki.linnakangas@i 1471 [ - + ]: 167562 : while (extraWaits-- > 0)
3431 tgl@sss.pgh.pa.us 1472 :UBC 0 : PGSemaphoreUnlock(proc->sem);
1473 : :
5209 heikki.linnakangas@i 1474 [ + + ]:CBC 167562 : if (mustwait)
1475 : : {
1476 : : /* Failed to get lock, so release interrupt holdoff */
1477 [ - + ]: 3159 : RESUME_INTERRUPTS();
1478 : : LOG_LWDEBUG("LWLockAcquireOrWait", lock, "failed");
1479 : : if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL_ENABLED())
1480 : : TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL(T_NAME(lock), mode);
1481 : : }
1482 : : else
1483 : : {
1484 : : LOG_LWDEBUG("LWLockAcquireOrWait", lock, "succeeded");
1485 : : /* Add lock to list of locks held by this backend */
4149 andres@anarazel.de 1486 : 164403 : held_lwlocks[num_held_lwlocks].lock = lock;
1487 : 164403 : held_lwlocks[num_held_lwlocks++].mode = mode;
1488 : : if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_ENABLED())
1489 : : TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT(T_NAME(lock), mode);
1490 : : }
1491 : :
5209 heikki.linnakangas@i 1492 : 167562 : return !mustwait;
1493 : : }
1494 : :
1495 : : /*
1496 : : * Does the lwlock in its current state need to wait for the variable value to
1497 : : * change?
1498 : : *
1499 : : * If we don't need to wait, and it's because the value of the variable has
1500 : : * changed, store the current value in newval.
1501 : : *
1502 : : * *result is set to true if the lock was free, and false otherwise.
1503 : : */
1504 : : static bool
1015 michael@paquier.xyz 1505 : 4896128 : LWLockConflictsWithVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 oldval,
1506 : : uint64 *newval, bool *result)
1507 : : {
1508 : : bool mustwait;
1509 : : uint64 value;
1510 : :
1511 : : /*
1512 : : * Test first to see if it the slot is free right now.
1513 : : *
1514 : : * XXX: the unique caller of this routine, WaitXLogInsertionsToFinish()
1515 : : * via LWLockWaitForVar(), uses an implied barrier with a spinlock before
1516 : : * this, so we don't need a memory barrier here as far as the current
1517 : : * usage is concerned. But that might not be safe in general.
1518 : : */
3931 andres@anarazel.de 1519 : 4896128 : mustwait = (pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE) != 0;
1520 : :
1521 [ + + ]: 4896128 : if (!mustwait)
1522 : : {
1523 : 3060428 : *result = true;
1524 : 3060428 : return false;
1525 : : }
1526 : :
1527 : 1835700 : *result = false;
1528 : :
1529 : : /*
1530 : : * Reading this value atomically is safe even on platforms where uint64
1531 : : * cannot be read without observing a torn value.
1532 : : */
1015 michael@paquier.xyz 1533 : 1835700 : value = pg_atomic_read_u64(valptr);
1534 : :
3931 andres@anarazel.de 1535 [ + + ]: 1835700 : if (value != oldval)
1536 : : {
1537 : 1524725 : mustwait = false;
1538 : 1524725 : *newval = value;
1539 : : }
1540 : : else
1541 : : {
1542 : 310975 : mustwait = true;
1543 : : }
1544 : :
1545 : 1835700 : return mustwait;
1546 : : }
1547 : :
1548 : : /*
1549 : : * LWLockWaitForVar - Wait until lock is free, or a variable is updated.
1550 : : *
1551 : : * If the lock is held and *valptr equals oldval, waits until the lock is
1552 : : * either freed, or the lock holder updates *valptr by calling
1553 : : * LWLockUpdateVar. If the lock is free on exit (immediately or after
1554 : : * waiting), returns true. If the lock is still held, but *valptr no longer
1555 : : * matches oldval, returns false and sets *newval to the current value in
1556 : : * *valptr.
1557 : : *
1558 : : * Note: this function ignores shared lock holders; if the lock is held
1559 : : * in shared mode, returns 'true'.
1560 : : *
1561 : : * Be aware that LWLockConflictsWithVar() does not include a memory barrier,
1562 : : * hence the caller of this function may want to rely on an explicit barrier or
1563 : : * an implied barrier via spinlock or LWLock to avoid memory ordering issues.
1564 : : */
1565 : : bool
1015 michael@paquier.xyz 1566 : 4585153 : LWLockWaitForVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 oldval,
1567 : : uint64 *newval)
1568 : : {
4428 heikki.linnakangas@i 1569 : 4585153 : PGPROC *proc = MyProc;
1570 : 4585153 : int extraWaits = 0;
1571 : 4585153 : bool result = false;
1572 : : #ifdef LWLOCK_STATS
1573 : : lwlock_stats *lwstats;
1574 : :
1575 : : lwstats = get_lwlock_stats_entry(lock);
1576 : : #endif
1577 : :
1578 : : PRINT_LWDEBUG("LWLockWaitForVar", lock, LW_WAIT_UNTIL_FREE);
1579 : :
1580 : : /*
1581 : : * Lock out cancel/die interrupts while we sleep on the lock. There is no
1582 : : * cleanup mechanism to remove us from the wait queue if we got
1583 : : * interrupted.
1584 : : */
1585 : 4585153 : HOLD_INTERRUPTS();
1586 : :
1587 : : /*
1588 : : * Loop here to check the lock's status after each time we are signaled.
1589 : : */
1590 : : for (;;)
1591 : 155096 : {
1592 : : bool mustwait;
1593 : :
3931 andres@anarazel.de 1594 : 4740249 : mustwait = LWLockConflictsWithVar(lock, valptr, oldval, newval,
1595 : : &result);
1596 : :
4428 heikki.linnakangas@i 1597 [ + + ]: 4740249 : if (!mustwait)
1598 : 4584370 : break; /* the lock was free or value didn't match */
1599 : :
1600 : : /*
1601 : : * Add myself to wait queue. Note that this is racy, somebody else
1602 : : * could wakeup before we're finished queuing. NB: We're using nearly
1603 : : * the same twice-in-a-row lock acquisition protocol as
1604 : : * LWLockAcquire(). Check its comments for details. The only
1605 : : * difference is that we also have to check the variable's values when
1606 : : * checking the state of the lock.
1607 : : */
4149 andres@anarazel.de 1608 : 155879 : LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);
1609 : :
1610 : : /*
1611 : : * Clear LW_FLAG_WAKE_IN_PROGRESS flag, to make sure we get woken up
1612 : : * as soon as the lock is released.
1613 : : */
112 andres@anarazel.de 1614 :GNC 155879 : pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_WAKE_IN_PROGRESS);
1615 : :
1616 : : /*
1617 : : * We're now guaranteed to be woken up if necessary. Recheck the lock
1618 : : * and variables state.
1619 : : */
3931 andres@anarazel.de 1620 :CBC 155879 : mustwait = LWLockConflictsWithVar(lock, valptr, oldval, newval,
1621 : : &result);
1622 : :
1623 : : /* Ok, no conflict after we queued ourselves. Undo queueing. */
4149 1624 [ + + ]: 155879 : if (!mustwait)
1625 : : {
1626 : : LOG_LWDEBUG("LWLockWaitForVar", lock, "free, undoing queue");
1627 : :
1628 : 783 : LWLockDequeueSelf(lock);
1629 : 783 : break;
1630 : : }
1631 : :
1632 : : /*
1633 : : * Wait until awakened.
1634 : : *
1635 : : * It is possible that we get awakened for a reason other than being
1636 : : * signaled by LWLockRelease. If so, loop back and wait again. Once
1637 : : * we've gotten the LWLock, re-increment the sema by the number of
1638 : : * additional signals received.
1639 : : */
1640 : : LOG_LWDEBUG("LWLockWaitForVar", lock, "waiting");
1641 : :
1642 : : #ifdef LWLOCK_STATS
1643 : : lwstats->block_count++;
1644 : : #endif
1645 : :
3708 rhaas@postgresql.org 1646 : 155096 : LWLockReportWaitStart(lock);
1647 : : if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
1648 : : TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), LW_EXCLUSIVE);
1649 : :
1650 : : for (;;)
1651 : : {
3431 tgl@sss.pgh.pa.us 1652 : 155096 : PGSemaphoreLock(proc->sem);
1262 andres@anarazel.de 1653 [ + - ]: 155096 : if (proc->lwWaiting == LW_WS_NOT_WAITING)
4428 heikki.linnakangas@i 1654 : 155096 : break;
4428 heikki.linnakangas@i 1655 :UBC 0 : extraWaits++;
1656 : : }
1657 : :
1658 : : #ifdef LOCK_DEBUG
1659 : : {
1660 : : /* not waiting anymore */
1661 : : uint32 nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);
1662 : :
1663 : : Assert(nwaiters < MAX_BACKENDS);
1664 : : }
1665 : : #endif
1666 : :
1667 : : if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
1668 : : TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), LW_EXCLUSIVE);
3708 rhaas@postgresql.org 1669 :CBC 155096 : LWLockReportWaitEnd();
1670 : :
1671 : : LOG_LWDEBUG("LWLockWaitForVar", lock, "awakened");
1672 : :
1673 : : /* Now loop back and check the status of the lock again. */
1674 : : }
1675 : :
1676 : : /*
1677 : : * Fix the process wait semaphore's count for any absorbed wakeups.
1678 : : */
4428 heikki.linnakangas@i 1679 [ - + ]: 4585153 : while (extraWaits-- > 0)
3431 tgl@sss.pgh.pa.us 1680 :UBC 0 : PGSemaphoreUnlock(proc->sem);
1681 : :
1682 : : /*
1683 : : * Now okay to allow cancel/die interrupts.
1684 : : */
4428 heikki.linnakangas@i 1685 [ - + ]:CBC 4585153 : RESUME_INTERRUPTS();
1686 : :
1687 : 4585153 : return result;
1688 : : }
1689 : :
1690 : :
1691 : : /*
1692 : : * LWLockUpdateVar - Update a variable and wake up waiters atomically
1693 : : *
1694 : : * Sets *valptr to 'val', and wakes up all processes waiting for us with
1695 : : * LWLockWaitForVar(). It first sets the value atomically and then wakes up
1696 : : * waiting processes so that any process calling LWLockWaitForVar() on the same
1697 : : * lock is guaranteed to see the new value, and act accordingly.
1698 : : *
1699 : : * The caller must be holding the lock in exclusive mode.
1700 : : */
1701 : : void
1015 michael@paquier.xyz 1702 : 2713952 : LWLockUpdateVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 val)
1703 : : {
1704 : : proclist_head wakeup;
1705 : : proclist_mutable_iter iter;
1706 : :
1707 : : PRINT_LWDEBUG("LWLockUpdateVar", lock, LW_EXCLUSIVE);
1708 : :
1709 : : /*
1710 : : * Note that pg_atomic_exchange_u64 is a full barrier, so we're guaranteed
1711 : : * that the variable is updated before waking up waiters.
1712 : : */
1713 : 2713952 : pg_atomic_exchange_u64(valptr, val);
1714 : :
3550 rhaas@postgresql.org 1715 : 2713952 : proclist_init(&wakeup);
1716 : :
3677 andres@anarazel.de 1717 : 2713952 : LWLockWaitListLock(lock);
1718 : :
4149 1719 [ - + ]: 2713952 : Assert(pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE);
1720 : :
1721 : : /*
1722 : : * See if there are any LW_WAIT_UNTIL_FREE waiters that need to be woken
1723 : : * up. They are always in the front of the queue.
1724 : : */
3550 rhaas@postgresql.org 1725 [ + + + + ]: 2714481 : proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
1726 : : {
1727 : 61782 : PGPROC *waiter = GetPGProcByNumber(iter.cur);
1728 : :
4149 andres@anarazel.de 1729 [ + + ]: 61782 : if (waiter->lwWaitMode != LW_WAIT_UNTIL_FREE)
1730 : 61253 : break;
1731 : :
3550 rhaas@postgresql.org 1732 : 529 : proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
1733 : 529 : proclist_push_tail(&wakeup, iter.cur, lwWaitLink);
1734 : :
1735 : : /* see LWLockWakeup() */
1262 andres@anarazel.de 1736 [ - + ]: 529 : Assert(waiter->lwWaiting == LW_WS_WAITING);
1737 [ + + ]: 529 : waiter->lwWaiting = LW_WS_PENDING_WAKEUP;
1738 : : }
1739 : :
1740 : : /* We are done updating shared state of the lock itself. */
3677 1741 : 2713952 : LWLockWaitListUnlock(lock);
1742 : :
1743 : : /*
1744 : : * Awaken any waiters I removed from the queue.
1745 : : */
3550 rhaas@postgresql.org 1746 [ + + + + : 2714481 : proclist_foreach_modify(iter, &wakeup, lwWaitLink)
+ + ]
1747 : : {
1748 : 529 : PGPROC *waiter = GetPGProcByNumber(iter.cur);
1749 : :
1750 : 529 : proclist_delete(&wakeup, iter.cur, lwWaitLink);
1751 : : /* check comment in LWLockWakeup() about this barrier */
4155 andres@anarazel.de 1752 : 529 : pg_write_barrier();
1262 1753 : 529 : waiter->lwWaiting = LW_WS_NOT_WAITING;
3431 tgl@sss.pgh.pa.us 1754 : 529 : PGSemaphoreUnlock(waiter->sem);
1755 : : }
4428 heikki.linnakangas@i 1756 : 2713952 : }
1757 : :
1758 : :
1759 : : /*
1760 : : * LWLockRelease - release a previously acquired lock
1761 : : *
1762 : : * NB: This will leave lock->owner pointing to the current backend (if
1763 : : * LOCK_DEBUG is set). This is somewhat intentional, as it makes it easier to
1764 : : * debug cases of missing wakeups during lock release.
1765 : : */
1766 : : void
110 andres@anarazel.de 1767 :GNC 222083727 : LWLockRelease(LWLock *lock)
1768 : : {
1769 : : LWLockMode mode;
1770 : : uint32 oldstate;
1771 : : bool check_waiters;
1772 : : int i;
1773 : :
1774 : : /*
1775 : : * Remove lock from list of locks held. Usually, but not always, it will
1776 : : * be the latest-acquired lock; so search array backwards.
1777 : : */
8958 bruce@momjian.us 1778 [ + - ]:CBC 251723069 : for (i = num_held_lwlocks; --i >= 0;)
4149 andres@anarazel.de 1779 [ + + ]: 251723069 : if (lock == held_lwlocks[i].lock)
8984 tgl@sss.pgh.pa.us 1780 : 222083727 : break;
1781 : :
1782 [ - + ]: 222083727 : if (i < 0)
3427 rhaas@postgresql.org 1783 [ # # ]:UBC 0 : elog(ERROR, "lock %s is not held", T_NAME(lock));
1784 : :
3437 sfrost@snowman.net 1785 :CBC 222083727 : mode = held_lwlocks[i].mode;
1786 : :
8984 tgl@sss.pgh.pa.us 1787 : 222083727 : num_held_lwlocks--;
1788 [ + + ]: 251723069 : for (; i < num_held_lwlocks; i++)
8958 bruce@momjian.us 1789 : 29639342 : held_lwlocks[i] = held_lwlocks[i + 1];
1790 : :
1791 : : PRINT_LWDEBUG("LWLockRelease", lock, mode);
1792 : :
1793 : : /*
1794 : : * Release my hold on lock, after that it can immediately be acquired by
1795 : : * others, even if we still have to wakeup other waiters.
1796 : : */
4149 andres@anarazel.de 1797 [ + + ]: 222083727 : if (mode == LW_EXCLUSIVE)
1798 : 126679783 : oldstate = pg_atomic_sub_fetch_u32(&lock->state, LW_VAL_EXCLUSIVE);
1799 : : else
1800 : 95403944 : oldstate = pg_atomic_sub_fetch_u32(&lock->state, LW_VAL_SHARED);
1801 : :
1802 : : /* nobody else can have that kind of lock */
1803 [ - + ]: 222083727 : Assert(!(oldstate & LW_VAL_EXCLUSIVE));
1804 : :
1805 : : if (TRACE_POSTGRESQL_LWLOCK_RELEASE_ENABLED())
1806 : : TRACE_POSTGRESQL_LWLOCK_RELEASE(T_NAME(lock));
1807 : :
1808 : : /*
1809 : : * Check if we're still waiting for backends to get scheduled, if so,
1810 : : * don't wake them up again.
1811 : : */
112 andres@anarazel.de 1812 [ + + ]:GNC 222083727 : if ((oldstate & LW_FLAG_HAS_WAITERS) &&
1813 [ + + ]: 319563 : !(oldstate & LW_FLAG_WAKE_IN_PROGRESS) &&
4149 andres@anarazel.de 1814 [ + + ]:CBC 168463 : (oldstate & LW_LOCK_MASK) == 0)
1815 : 166523 : check_waiters = true;
1816 : : else
1817 : 221917204 : check_waiters = false;
1818 : :
1819 : : /*
1820 : : * As waking up waiters requires the spinlock to be acquired, only do so
1821 : : * if necessary.
1822 : : */
1823 [ + + ]: 222083727 : if (check_waiters)
1824 : : {
1825 : : /* XXX: remove before commit? */
1826 : : LOG_LWDEBUG("LWLockRelease", lock, "releasing waiters");
1827 : 166523 : LWLockWakeup(lock);
1828 : : }
1829 : :
1830 : : /*
1831 : : * Now okay to allow cancel/die interrupts.
1832 : : */
8984 tgl@sss.pgh.pa.us 1833 [ - + ]: 222083727 : RESUME_INTERRUPTS();
1834 : 222083727 : }
1835 : :
1836 : : /*
1837 : : * LWLockReleaseClearVar - release a previously acquired lock, reset variable
1838 : : */
1839 : : void
1015 michael@paquier.xyz 1840 : 24706732 : LWLockReleaseClearVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 val)
1841 : : {
1842 : : /*
1843 : : * Note that pg_atomic_exchange_u64 is a full barrier, so we're guaranteed
1844 : : * that the variable is updated before releasing the lock.
1845 : : */
1846 : 24706732 : pg_atomic_exchange_u64(valptr, val);
1847 : :
3931 andres@anarazel.de 1848 : 24706732 : LWLockRelease(lock);
1849 : 24706732 : }
1850 : :
1851 : :
1852 : : /*
1853 : : * LWLockReleaseAll - release all currently-held locks
1854 : : *
1855 : : * Used to clean up after ereport(ERROR). An important difference between this
1856 : : * function and retail LWLockRelease calls is that InterruptHoldoffCount is
1857 : : * unchanged by this operation. This is necessary since InterruptHoldoffCount
1858 : : * has been set to an appropriate level earlier in error recovery. We could
1859 : : * decrement it below zero if we allow it to drop for each released lock!
1860 : : *
1861 : : * Note that this function must be safe to call even before the LWLock
1862 : : * subsystem has been initialized (e.g., during early startup failures).
1863 : : * In that case, num_held_lwlocks will be 0 and we do nothing.
1864 : : */
1865 : : void
8984 tgl@sss.pgh.pa.us 1866 : 117045 : LWLockReleaseAll(void)
1867 : : {
1868 [ + + ]: 117151 : while (num_held_lwlocks > 0)
1869 : : {
1870 : 106 : HOLD_INTERRUPTS(); /* match the upcoming RESUME_INTERRUPTS */
1871 : :
4149 andres@anarazel.de 1872 : 106 : LWLockRelease(held_lwlocks[num_held_lwlocks - 1].lock);
1873 : : }
1874 : :
109 amitlan@postgresql.o 1875 [ - + ]: 117045 : Assert(num_held_lwlocks == 0);
8984 tgl@sss.pgh.pa.us 1876 : 117045 : }
1877 : :
1878 : :
1879 : : /*
1880 : : * LWLockHeldByMe - test whether my process holds a lock in any mode
1881 : : *
1882 : : * This is meant as debug support only.
1883 : : */
1884 : : bool
1324 pg@bowt.ie 1885 : 5199837 : LWLockHeldByMe(LWLock *lock)
1886 : : {
1887 : : int i;
1888 : :
7998 tgl@sss.pgh.pa.us 1889 [ + + ]: 39562617 : for (i = 0; i < num_held_lwlocks; i++)
1890 : : {
1324 pg@bowt.ie 1891 [ + + ]: 38268890 : if (held_lwlocks[i].lock == lock)
7998 tgl@sss.pgh.pa.us 1892 : 3906110 : return true;
1893 : : }
1894 : 1293727 : return false;
1895 : : }
1896 : :
1897 : : /*
1898 : : * LWLockAnyHeldByMe - test whether my process holds any of an array of locks
1899 : : *
1900 : : * This is meant as debug support only.
1901 : : */
1902 : : bool
1324 pg@bowt.ie 1903 : 1481201 : LWLockAnyHeldByMe(LWLock *lock, int nlocks, size_t stride)
1904 : : {
1905 : : char *held_lock_addr;
1906 : : char *begin;
1907 : : char *end;
1908 : : int i;
1909 : :
1910 : 1481201 : begin = (char *) lock;
1394 tmunro@postgresql.or 1911 : 1481201 : end = begin + nlocks * stride;
1912 [ + + ]: 1482957 : for (i = 0; i < num_held_lwlocks; i++)
1913 : : {
1914 : 1756 : held_lock_addr = (char *) held_lwlocks[i].lock;
1915 [ + + - + ]: 1756 : if (held_lock_addr >= begin &&
1394 tmunro@postgresql.or 1916 :UBC 0 : held_lock_addr < end &&
1917 [ # # ]: 0 : (held_lock_addr - begin) % stride == 0)
1918 : 0 : return true;
1919 : : }
1394 tmunro@postgresql.or 1920 :CBC 1481201 : return false;
1921 : : }
1922 : :
1923 : : /*
1924 : : * LWLockHeldByMeInMode - test whether my process holds a lock in given mode
1925 : : *
1926 : : * This is meant as debug support only.
1927 : : */
1928 : : bool
1324 pg@bowt.ie 1929 : 4091775 : LWLockHeldByMeInMode(LWLock *lock, LWLockMode mode)
1930 : : {
1931 : : int i;
1932 : :
3529 simon@2ndQuadrant.co 1933 [ + - ]: 4094015 : for (i = 0; i < num_held_lwlocks; i++)
1934 : : {
1324 pg@bowt.ie 1935 [ + + + - ]: 4094015 : if (held_lwlocks[i].lock == lock && held_lwlocks[i].mode == mode)
3529 simon@2ndQuadrant.co 1936 : 4091775 : return true;
1937 : : }
3529 simon@2ndQuadrant.co 1938 :UBC 0 : return false;
1939 : : }
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