* IDENTIFICATION
* src/backend/storage/lmgr/lwlock.c
*
+ * NOTES:
+ *
+ * This used to be a pretty straight forward reader-writer lock
+ * implementation, in which the internal state was protected by a
+ * spinlock. Unfortunately the overhead of taking the spinlock proved to be
+ * too high for workloads/locks that were locked in shared mode very
+ * frequently.
+ * Thus a new implementation was devised that provides wait-free shared lock
+ * acquiration for locks that aren't exclusively locked.
+ *
+ * The basic idea is to have a single atomic variable 'lockcount' instead of
+ * the formerly separate shared and exclusive counters and to use an atomic
+ * increment to acquire the lock. That's fairly easy to do for rw-spinlocks,
+ * but a lot harder for something like LWLocks that want to wait in the OS.
+ *
+ * For exlusive lock acquisition we use an atomic compare-and-exchange on the
+ * lockcount variable swapping in EXCLUSIVE_LOCK/1<<31-1/0x7FFFFFFF if and only
+ * if the current value of lockcount is 0. If the swap was not successfull, we
+ * have to wait.
+ *
+ * For shared lock acquisition we use an atomic add (lock xadd) to the
+ * lockcount variable to add 1. If the value is bigger than EXCLUSIVE_LOCK we
+ * know that somebody actually has an exclusive lock, and we back out by
+ * atomically decrementing by 1 again. If so, we have to wait for the exlusive
+ * locker to release the lock.
+ *
+ * To release the lock we use an atomic decrement to release the lock. If the
+ * new value is zero (we get that atomically), we know we have to release
+ * waiters.
+ *
+ * The attentive reader probably might have noticed that naively doing the
+ * above has two glaring race conditions:
+ *
+ * 1) too-quick-for-queueing: We try to lock using the atomic operations and
+ * notice that we have to wait. Unfortunately until we have finished queuing,
+ * the former locker very well might have already finished it's work. That's
+ * problematic because we're now stuck waiting inside the OS.
+ *
+ * 2) spurious failed locks: Due to the logic of backing out of shared
+ * locks after we unconditionally added a 1 to lockcount, we might have
+ * prevented another exclusive locker from getting the lock:
+ * 1) Session A: LWLockAcquire(LW_EXCLUSIVE) - success
+ * 2) Session B: LWLockAcquire(LW_SHARED) - lockcount += 1
+ * 3) Session B: LWLockAcquire(LW_SHARED) - oops, bigger than EXCLUSIVE_LOCK
+ * 4) Session A: LWLockRelease()
+ * 5) Session C: LWLockAcquire(LW_EXCLUSIVE) - check if lockcount = 0, no. wait.
+ * 6) Session B: LWLockAcquire(LW_SHARED) - lockcount -= 1
+ * 7) Session B: LWLockAcquire(LW_SHARED) - wait
+ *
+ * So we'd now have both B) and C) waiting on a lock that nobody is holding
+ * anymore. Not good.
+ *
+ * To mitigate those races we use a two phased attempt at locking:
+ * Phase 1: * Try to do it atomically, if we succeed, nice
+ * Phase 2: Add us too the waitqueue of the lock
+ * Phase 3: Try to grab the lock again, if we succeed, remove ourselves from
+ * the queue
+ * Phase 4: Sleep till wakeup, goto Phase 1
+ *
+ * This protects us against both problems from above:
+ * 1) Nobody can release too quick, before we're queued, since after Phase 2
+ * we're already queued.
+ * 2) If somebody spuriously got blocked from acquiring the lock, they will
+ * get queued in Phase 2 and we can wake them up if neccessary or they will
+ * have gotten the lock in Phase 3.
+ *
+ * There above algorithm only works for LWLockAcquire, not directly for
+ * LWLockAcquireConditional where we don't want to wait. In that case we just
+ * need to retry acquiring the lock until we're sure we didn't disturb anybody
+ * in doing so.
+ *
+ * TODO:
+ * - decide if we need a spinlock fallback
+ * - expand documentation
+ * - make LWLOCK_STATS do something sensible again
+ * - make LOCK_DEBUG output nicer
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "replication/slot.h"
-#include "storage/barrier.h"
#include "storage/ipc.h"
#include "storage/predicate.h"
#include "storage/proc.h"
/* We use the ShmemLock spinlock to protect LWLockAssign */
extern slock_t *ShmemLock;
+#define EXCLUSIVE_LOCK (((uint32) 1) << (31 - 1))
+/* must be greater than MAX_BACKENDS */
+#define SHARED_LOCK_MASK (~EXCLUSIVE_LOCK)
+
/*
* This is indexed by tranche ID and stores metadata for all tranches known
* to the current backend.
*/
#define MAX_SIMUL_LWLOCKS 100
+typedef struct LWLockHandle
+{
+ LWLock *lock;
+ LWLockMode mode;
+} LWLockHandle;
+
static int num_held_lwlocks = 0;
-static LWLock *held_lwlocks[MAX_SIMUL_LWLOCKS];
+static LWLockHandle held_lwlocks[MAX_SIMUL_LWLOCKS];
static int lock_addin_request = 0;
static bool lock_addin_request_allowed = true;
{
lwlock_stats_key key;
int sh_acquire_count;
+ int sh_attempt_backout;
int ex_acquire_count;
+ int ex_race;
int block_count;
+ int dequeue_self_count;
int spin_delay_count;
} lwlock_stats;
bool Trace_lwlocks = false;
inline static void
-PRINT_LWDEBUG(const char *where, const LWLock *lock)
+PRINT_LWDEBUG(const char *where, const LWLock *lock, LWLockMode mode)
{
if (Trace_lwlocks)
- elog(LOG, "%s(%s %d): excl %d shared %d rOK %d",
+ {
+ uint32 lockcount = pg_atomic_read_u32(&lock->lockcount);
+
+ elog(LOG, "%d: %s(%s %d): excl %u shared %u waiters %u rOK %d\n",
+ MyProcPid,
where, T_NAME(lock), T_ID(lock),
- (int) lock->exclusive, lock->shared,
+ lockcount >= EXCLUSIVE_LOCK,
+ lockcount & SHARED_LOCK_MASK,
+ pg_atomic_read_u32(&lock->nwaiters),
(int) lock->releaseOK);
+ }
}
inline static void
LOG_LWDEBUG(const char *where, const char *name, int index, const char *msg)
{
if (Trace_lwlocks)
- elog(LOG, "%s(%s %d): %s", where, name, index, msg);
+ elog(LOG, "%d: %s(%s %d): %s\n", MyProcPid, where, name, index, msg);
}
#else /* not LOCK_DEBUG */
-#define PRINT_LWDEBUG(a,b)
+#define PRINT_LWDEBUG(a,b,c)
#define LOG_LWDEBUG(a,b,c,d)
#endif /* LOCK_DEBUG */
while ((lwstats = (lwlock_stats *) hash_seq_search(&scan)) != NULL)
{
fprintf(stderr,
- "PID %d lwlock %s %d: shacq %u exacq %u blk %u spindelay %u\n",
+ "PID %d lwlock %s %d: shacq %u exacq %u blk %u spindelay %u, backout %u, ex race %u, dequeue self %u\n",
MyProcPid, LWLockTrancheArray[lwstats->key.tranche]->name,
lwstats->key.instance, lwstats->sh_acquire_count,
lwstats->ex_acquire_count, lwstats->block_count,
- lwstats->spin_delay_count);
+ lwstats->spin_delay_count, lwstats->sh_attempt_backout,
+ lwstats->ex_race, lwstats->dequeue_self_count);
}
LWLockRelease(&MainLWLockArray[0].lock);
if (!found)
{
lwstats->sh_acquire_count = 0;
+ lwstats->sh_attempt_backout = 0;
lwstats->ex_acquire_count = 0;
+ lwstats->ex_race = 0;
lwstats->block_count = 0;
+ lwstats->dequeue_self_count = 0;
lwstats->spin_delay_count = 0;
}
return lwstats;
{
SpinLockInit(&lock->mutex);
lock->releaseOK = true;
- lock->exclusive = 0;
- lock->shared = 0;
+ pg_atomic_init_u32(&lock->lockcount, 0);
+ pg_atomic_init_u32(&lock->nwaiters, 0);
lock->tranche = tranche_id;
dlist_init(&lock->waiters);
}
+/*
+ * Internal function handling the atomic manipulation of lock->lockcount.
+ *
+ * 'double_check' = true means that we try to check more carefully
+ * against causing somebody else to spuriously believe the lock is
+ * already taken, although we're just about to back out of it.
+ */
+static inline bool
+LWLockAttemptLock(LWLock* l, LWLockMode mode, bool double_check, bool *potentially_spurious)
+{
+ bool mustwait;
+ uint32 oldstate;
+#ifdef LWLOCK_STATS
+ lwlock_stats *lwstats;
+ lwstats = get_lwlock_stats_entry(l);
+#endif
+
+ Assert(mode == LW_EXCLUSIVE || mode == LW_SHARED);
+
+ *potentially_spurious = false;
+
+ if (mode == LW_EXCLUSIVE)
+ {
+ uint32 expected;
+
+ /* check without CAS first; it's way cheaper, frequently locked otherwise */
+ expected = pg_atomic_read_u32(&l->lockcount);
+
+ Assert(expected < EXCLUSIVE_LOCK + (1 << 16));
+
+ if (expected != 0)
+ mustwait = true;
+ else if (!pg_atomic_compare_exchange_u32(&l->lockcount,
+ &expected, EXCLUSIVE_LOCK))
+ {
+ /*
+ * ok, no can do. Between the pg_atomic_read() above and the
+ * CAS somebody else acquired the lock.
+ */
+ mustwait = true;
+ Assert(expected < EXCLUSIVE_LOCK + (1 << 16));
+ }
+ else
+ {
+ /* yipeyyahee */
+ mustwait = false;
+#ifdef LOCK_DEBUG
+ l->owner = MyProc;
+#endif
+ Assert(expected == 0);
+ }
+ }
+ else
+ {
+ /*
+ * If requested by caller, do an unlocked check first. This is useful
+ * if potentially spurious results have a noticeable cost.
+ */
+ if (double_check)
+ {
+ if (pg_atomic_read_u32(&l->lockcount) >= EXCLUSIVE_LOCK)
+ {
+ mustwait = true;
+ goto out;
+ }
+ }
+
+ /*
+ * Acquire the share lock unconditionally using an atomic addition. We
+ * might have to back out again if it turns out somebody else has an
+ * exclusive lock.
+ */
+ oldstate = pg_atomic_fetch_add_u32(&l->lockcount, 1);
+
+ if (oldstate >= EXCLUSIVE_LOCK)
+ {
+ /*
+ * Ok, somebody else holds the lock exclusively. We need to back
+ * away from the shared lock, since we don't actually hold it right
+ * now. Since there's a window between lockcount += 1 and lockcount
+ * -= 1, the previous exclusive locker could have released and
+ * another exclusive locker could have seen our +1. We need to
+ * signal that to the upper layers so they can deal with the race
+ * condition.
+ */
+
+ /*
+ * FIXME: check return value if (double_check), it's not
+ * spurious if still exclusively locked.
+ */
+ pg_atomic_fetch_sub_u32(&l->lockcount, 1);
+
+
+ mustwait = true;
+ *potentially_spurious = true;
+#ifdef LWLOCK_STATS
+ lwstats->sh_attempt_backout++;
+#endif
+ }
+ else
+ {
+ /* yipeyyahee */
+ mustwait = false;
+ }
+ }
+
+out:
+ return mustwait;
+}
+
+/*
+ * Wakeup all the lockers that currently have a chance to run.
+ */
+static void
+LWLockWakeup(LWLock *lock, LWLockMode mode)
+{
+ bool releaseOK;
+ bool wokeup_somebody = false;
+ dlist_head wakeup;
+ dlist_mutable_iter iter;
+#ifdef LWLOCK_STATS
+ lwlock_stats *lwstats;
+ lwstats = get_lwlock_stats_entry(l);
+#endif
+
+ dlist_init(&wakeup);
+
+ /* remove the to-be-awakened PGPROCs from the queue */
+ releaseOK = true;
+
+ /* Acquire mutex. Time spent holding mutex should be short! */
+#ifdef LWLOCK_STATS
+ lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
+#else
+ SpinLockAcquire(&lock->mutex);
+#endif
+
+ /*
+ * We're still waiting for backends to get scheduled, don't wake them up
+ * again.
+ */
+ if (!lock->releaseOK)
+ {
+ SpinLockRelease(&lock->mutex);
+ PRINT_LWDEBUG("LWLockRelease skip releaseok", lock, mode);
+ return;
+ }
+
+ dlist_foreach_modify(iter, (dlist_head *) &lock->waiters)
+ {
+ PGPROC *waiter = dlist_container(PGPROC, lwWaitLink, iter.cur);
+
+ if (wokeup_somebody && waiter->lwWaitMode == LW_EXCLUSIVE)
+ continue;
+
+ dlist_delete(&waiter->lwWaitLink);
+ dlist_push_tail(&wakeup, &waiter->lwWaitLink);
+
+ if (waiter->lwWaitMode != LW_WAIT_UNTIL_FREE)
+ {
+ /*
+ * Prevent additional wakeups until retryer gets to run. Backends
+ * that are just waiting for the lock to become free don't retry
+ * automatically.
+ */
+ releaseOK = false;
+ /*
+ * Don't wakeup (further) exclusive locks.
+ */
+ wokeup_somebody = true;
+ }
+
+ /*
+ * Once we've woken up an exclusive lock, there's no point in waking
+ * up anybody else.
+ */
+ if(waiter->lwWaitMode == LW_EXCLUSIVE)
+ break;
+ }
+ lock->releaseOK = releaseOK;
+
+
+ /* We are done updating shared state of the lock queue. */
+ SpinLockRelease(&lock->mutex);
+
+ /*
+ * Awaken any waiters I removed from the queue.
+ */
+ dlist_foreach_modify(iter, &wakeup)
+ {
+ PGPROC *waiter = dlist_container(PGPROC, lwWaitLink, iter.cur);
+
+ LOG_LWDEBUG("LWLockRelease", T_NAME(l), T_ID(l), "release waiter");
+ dlist_delete(&waiter->lwWaitLink);
+ pg_write_barrier();
+ waiter->lwWaiting = false;
+ PGSemaphoreUnlock(&waiter->sem);
+ }
+}
+
+/*
+ * Add ourselves to the end of the queue. Mode can be LW_WAIT_UNTIL_FREE here!
+ */
+static inline void
+LWLockQueueSelf(LWLock *lock, LWLockMode mode)
+{
+#ifdef LWLOCK_STATS
+ lwlock_stats *lwstats;
+ lwstats = get_lwlock_stats_entry(l);
+#endif
+
+ /*
+ * If we don't have a PGPROC structure, there's no way to wait. This
+ * should never occur, since MyProc should only be null during shared
+ * memory initialization.
+ */
+ if (MyProc == NULL)
+ elog(PANIC, "cannot wait without a PGPROC structure");
+
+ pg_atomic_fetch_add_u32(&lock->nwaiters, 1);
+
+#ifdef LWLOCK_STATS
+ lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
+#else
+ SpinLockAcquire(&lock->mutex);
+#endif
+
+ if (MyProc->lwWaiting)
+ elog(PANIC, "queueing for lock while waiting on another one");
+
+ MyProc->lwWaiting = true;
+ MyProc->lwWaitMode = mode;
+
+ /* LW_WAIT_UNTIL_FREE waiters are always at the front of the queue */
+ if (mode == LW_WAIT_UNTIL_FREE)
+ dlist_push_head((dlist_head *) &lock->waiters, &MyProc->lwWaitLink);
+ else
+ dlist_push_tail((dlist_head *) &lock->waiters, &MyProc->lwWaitLink);
+
+ /* Can release the mutex now */
+ SpinLockRelease(&lock->mutex);
+}
+
+/*
+ * Remove ourselves from the waitlist. This is used if we queued ourselves
+ * because we thought we needed to sleep but, after further checking, we
+ * discovered that we don't actually need to do so. Somebody else might have
+ * already woken us up though, in that case return false.
+ */
+static inline bool
+LWLockDequeueSelf(LWLock *lock)
+{
+ bool found = false;
+ dlist_mutable_iter iter;
+
+#ifdef LWLOCK_STATS
+ lwlock_stats *lwstats;
+ lwstats = get_lwlock_stats_entry(l);
+#endif
+
+#ifdef LWLOCK_STATS
+ lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
+#else
+ SpinLockAcquire(&lock->mutex);
+#endif
+
+ /* need to iterate, somebody else could have unqueued us */
+ dlist_foreach_modify(iter, (dlist_head *) &lock->waiters)
+ {
+ PGPROC *proc = dlist_container(PGPROC, lwWaitLink, iter.cur);
+ if (proc == MyProc)
+ {
+ found = true;
+ dlist_delete(&proc->lwWaitLink);
+ break;
+ }
+ }
+
+ /* clear waiting state again, nice for debugging */
+ if (found)
+ MyProc->lwWaiting = false;
+
+ SpinLockRelease(&lock->mutex);
+
+ pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);
+ return found;
+}
/*
* LWLockAcquire - acquire a lightweight lock in the specified mode
LWLockAcquireCommon(LWLock *lock, LWLockMode mode, uint64 *valptr, uint64 val)
{
PGPROC *proc = MyProc;
- bool retry = false;
bool result = true;
int extraWaits = 0;
+ bool potentially_spurious;
+
#ifdef LWLOCK_STATS
lwlock_stats *lwstats;
#endif
- PRINT_LWDEBUG("LWLockAcquire", lock);
+ AssertArg(mode == LW_SHARED || mode == LW_EXCLUSIVE);
+
+ PRINT_LWDEBUG("LWLockAcquire", lock, mode);
#ifdef LWLOCK_STATS
lwstats = get_lwlock_stats_entry(lock);
{
bool mustwait;
- /* Acquire mutex. Time spent holding mutex should be short! */
-#ifdef LWLOCK_STATS
- lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
-#else
- SpinLockAcquire(&lock->mutex);
-#endif
+ /*
+ * try to grab the lock the first time, we're not in the waitqueue yet.
+ */
+ mustwait = LWLockAttemptLock(lock, mode, false, &potentially_spurious);
- /* If retrying, allow LWLockRelease to release waiters again */
- if (retry)
- lock->releaseOK = true;
+ if (!mustwait)
+ {
+ LOG_LWDEBUG("LWLockAcquire", T_NAME(l), T_ID(l), "success");
+ break; /* got the lock */
+ }
- /* If I can get the lock, do so quickly. */
- if (mode == LW_EXCLUSIVE)
+ /*
+ * Ok, at this point we couldn't grab the lock on the first try. We
+ * cannot simply queue ourselves to the end of the list and wait to be
+ * woken up because by now the lock could long have been released.
+ * Instead add us to the queue and try to grab the lock again. If we
+ * succeed we need to revert the queuing and be happy, otherwise we
+ * recheck the lock. If we still couldn't grab it, we know that the
+ * other lock will see our queue entries when releasing since they
+ * existed before we checked for the lock.
+ * FIXME: add note referring to overall notes
+ */
+
+ /* add to the queue */
+ LWLockQueueSelf(lock, mode);
+
+ /* we're now guaranteed to be woken up if necessary */
+ mustwait = LWLockAttemptLock(lock, mode, false, &potentially_spurious);
+
+ /* ok, grabbed the lock the second time round, need to undo queueing */
+ if (!mustwait)
{
- if (lock->exclusive == 0 && lock->shared == 0)
+#ifdef LWLOCK_STATS
+ lwstats->dequeue_self_count++;
+#endif
+ if (!LWLockDequeueSelf(lock))
{
- lock->exclusive++;
- mustwait = false;
+ /*
+ * Somebody else dequeued us and has or will wake us up. Wait
+ * for the correct wakeup, otherwise our ->lwWaiting would get
+ * reset at some inconvenient point later, and releaseOk
+ * wouldn't be managed correctly.
+ */
+ for (;;)
+ {
+ PGSemaphoreLock(&proc->sem, false);
+ if (!proc->lwWaiting)
+ break;
+ extraWaits++;
+ }
+ /*
+ * Reset releaseOk - if somebody woke us they'll have set it
+ * to false.
+ */
+ SpinLockAcquire(&lock->mutex);
+ lock->releaseOK = true;
+ SpinLockRelease(&lock->mutex);
}
- else
- mustwait = true;
+ PRINT_LWDEBUG("LWLockAcquire success: undo queue", lock, mode);
+ break;
}
else
{
- if (lock->exclusive == 0)
- {
- lock->shared++;
- mustwait = false;
- }
- else
- mustwait = true;
+ PRINT_LWDEBUG("LWLockAcquire waiting 4", lock, mode);
}
- if (!mustwait)
- break; /* got the lock */
-
/*
- * Add myself to wait queue.
- *
- * If we don't have a PGPROC structure, there's no way to wait. This
- * should never occur, since MyProc should only be null during shared
- * memory initialization.
+ * NB: There's no need to deal with spurious lock attempts
+ * here. Anyone we prevented from acquiring the lock will
+ * enqueue themselves using the same protocol we used here.
*/
- if (proc == NULL)
- elog(PANIC, "cannot wait without a PGPROC structure");
-
- proc->lwWaiting = true;
- proc->lwWaitMode = mode;
- dlist_push_head((dlist_head *) &lock->waiters, &proc->lwWaitLink);
-
- /* Can release the mutex now */
- SpinLockRelease(&lock->mutex);
/*
* Wait until awakened.
LOG_LWDEBUG("LWLockAcquire", T_NAME(lock), T_ID(lock), "awakened");
+ /* not waiting anymore */
+ pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);
+
/* Now loop back and try to acquire lock again. */
- retry = true;
+ SpinLockAcquire(&lock->mutex);
+ lock->releaseOK = true;
+ SpinLockRelease(&lock->mutex);
+
result = false;
}
if (valptr)
*valptr = val;
- /* We are done updating shared state of the lock itself. */
- SpinLockRelease(&lock->mutex);
-
TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), T_ID(lock), mode);
/* Add lock to list of locks held by this backend */
- held_lwlocks[num_held_lwlocks++] = lock;
+ held_lwlocks[num_held_lwlocks].lock = lock;
+ held_lwlocks[num_held_lwlocks++].mode = mode;
/*
* Fix the process wait semaphore's count for any absorbed wakeups.
LWLockConditionalAcquire(LWLock *lock, LWLockMode mode)
{
bool mustwait;
+ bool potentially_spurious;
+
+ AssertArg(mode == LW_SHARED || mode == LW_EXCLUSIVE);
- PRINT_LWDEBUG("LWLockConditionalAcquire", lock);
+ PRINT_LWDEBUG("LWLockConditionalAcquire", l, mode);
/* Ensure we will have room to remember the lock */
if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
*/
HOLD_INTERRUPTS();
- /* Acquire mutex. Time spent holding mutex should be short! */
- SpinLockAcquire(&lock->mutex);
-
- /* If I can get the lock, do so quickly. */
- if (mode == LW_EXCLUSIVE)
- {
- if (lock->exclusive == 0 && lock->shared == 0)
- {
- lock->exclusive++;
- mustwait = false;
- }
- else
- mustwait = true;
- }
- else
- {
- if (lock->exclusive == 0)
- {
- lock->shared++;
- mustwait = false;
- }
- else
- mustwait = true;
- }
-
- /* We are done updating shared state of the lock itself. */
- SpinLockRelease(&lock->mutex);
+retry:
+ /*
+ * passing 'true' to check more carefully to avoid potential
+ * spurious acquisitions
+ */
+ mustwait = LWLockAttemptLock(lock, mode, true, &potentially_spurious);
if (mustwait)
{
/* Failed to get lock, so release interrupt holdoff */
RESUME_INTERRUPTS();
+
LOG_LWDEBUG("LWLockConditionalAcquire",
T_NAME(lock), T_ID(lock), "failed");
- TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL(T_NAME(lock),
- T_ID(lock), mode);
+ TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL(T_NAME(lock), T_ID(lock), mode);
+
+ /*
+ * We ran into an exclusive lock and might have blocked another
+ * exclusive lock from taking a shot because it took a time to back
+ * off. Retry till we are either sure we didn't block somebody (because
+ * somebody else certainly has the lock) or till we got it.
+ *
+ * We cannot rely on the two-step lock-acquisition protocol as in
+ * LWLockAcquire because we're not using it.
+ */
+ if (potentially_spurious)
+ {
+ SPIN_DELAY();
+ goto retry;
+ }
}
else
{
/* Add lock to list of locks held by this backend */
- held_lwlocks[num_held_lwlocks++] = lock;
+ held_lwlocks[num_held_lwlocks].lock = lock;
+ held_lwlocks[num_held_lwlocks++].mode = mode;
TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE(T_NAME(lock), T_ID(lock), mode);
}
-
return !mustwait;
}
PGPROC *proc = MyProc;
bool mustwait;
int extraWaits = 0;
+ bool potentially_spurious_first;
+ bool potentially_spurious_second;
#ifdef LWLOCK_STATS
lwlock_stats *lwstats;
#endif
- PRINT_LWDEBUG("LWLockAcquireOrWait", lock);
+ Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);
+
+ PRINT_LWDEBUG("LWLockAcquireOrWait", lock, mode);
#ifdef LWLOCK_STATS
lwstats = get_lwlock_stats_entry(lock);
*/
HOLD_INTERRUPTS();
- /* Acquire mutex. Time spent holding mutex should be short! */
- SpinLockAcquire(&lock->mutex);
-
- /* If I can get the lock, do so quickly. */
- if (mode == LW_EXCLUSIVE)
- {
- if (lock->exclusive == 0 && lock->shared == 0)
- {
- lock->exclusive++;
- mustwait = false;
- }
- else
- mustwait = true;
- }
- else
- {
- if (lock->exclusive == 0)
- {
- lock->shared++;
- mustwait = false;
- }
- else
- mustwait = true;
- }
+ /*
+ * NB: We're using nearly the same twice-in-a-row lock acquisition
+ * protocol as LWLockAcquire(). Check its comments for details.
+ */
+ mustwait = LWLockAttemptLock(lock, mode, false, &potentially_spurious_first);
if (mustwait)
{
- /*
- * Add myself to wait queue.
- *
- * If we don't have a PGPROC structure, there's no way to wait. This
- * should never occur, since MyProc should only be null during shared
- * memory initialization.
- */
- if (proc == NULL)
- elog(PANIC, "cannot wait without a PGPROC structure");
-
- proc->lwWaiting = true;
- proc->lwWaitMode = LW_WAIT_UNTIL_FREE;
- dlist_push_head((dlist_head *) &lock->waiters, &proc->lwWaitLink);
+ LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);
- /* Can release the mutex now */
- SpinLockRelease(&lock->mutex);
+ mustwait = LWLockAttemptLock(lock, mode, false, &potentially_spurious_second);
- /*
- * Wait until awakened. Like in LWLockAcquire, be prepared for bogus
- * wakups, because we share the semaphore with ProcWaitForSignal.
- */
- LOG_LWDEBUG("LWLockAcquireOrWait", T_NAME(lock), T_ID(lock),
- "waiting");
+ if (mustwait)
+ {
+ /*
+ * Wait until awakened. Like in LWLockAcquire, be prepared for bogus
+ * wakups, because we share the semaphore with ProcWaitForSignal.
+ */
+ LOG_LWDEBUG("LWLockAcquireOrWait", T_NAME(lock), T_ID(lock),
+ "waiting");
#ifdef LWLOCK_STATS
- lwstats->block_count++;
+ lwstats->block_count++;
#endif
+ TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(l), T_ID(l), mode);
- TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), T_ID(lock), mode);
+ for (;;)
+ {
+ /* "false" means cannot accept cancel/die interrupt here. */
+ PGSemaphoreLock(&proc->sem, false);
+ if (!proc->lwWaiting)
+ break;
+ extraWaits++;
+ }
+ pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);
- for (;;)
+ TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), T_ID(lock), mode);
+ }
+ else
{
- /* "false" means cannot accept cancel/die interrupt here. */
- PGSemaphoreLock(&proc->sem, false);
- if (!proc->lwWaiting)
- break;
- extraWaits++;
+ /* got lock in the second attempt, undo queueing */
+ if (!LWLockDequeueSelf(lock))
+ {
+ for (;;)
+ {
+ PGSemaphoreLock(&proc->sem, false);
+ if (!proc->lwWaiting)
+ break;
+ extraWaits++;
+ }
+ }
}
-
- TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), T_ID(lock), mode);
-
- LOG_LWDEBUG("LWLockAcquireOrWait", T_NAME(lock), T_ID(lock),
- "awakened");
- }
- else
- {
- /* We are done updating shared state of the lock itself. */
- SpinLockRelease(&lock->mutex);
}
/*
}
else
{
+ LOG_LWDEBUG("LWLockAcquireOrWait", T_NAME(lock), T_ID(lock), "succeeded");
/* Add lock to list of locks held by this backend */
- held_lwlocks[num_held_lwlocks++] = lock;
- TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT(T_NAME(lock), T_ID(lock),
- mode);
+ held_lwlocks[num_held_lwlocks].lock = lock;
+ held_lwlocks[num_held_lwlocks++].mode = mode;
+ TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT(T_NAME(lock), T_ID(lock), mode);
}
return !mustwait;
lwlock_stats *lwstats;
#endif
- PRINT_LWDEBUG("LWLockWaitForVar", lock);
+ PRINT_LWDEBUG("LWLockWaitForVar", lock, LW_EXCLUSIVE);
#ifdef LWLOCK_STATS
lwstats = get_lwlock_stats_entry(lock);
* barrier here as far as the current usage is concerned. But that might
* not be safe in general.
*/
- if (lock->exclusive == 0)
+ if (pg_atomic_read_u32(&lock->lockcount) == 0)
return true;
/*
bool mustwait;
uint64 value;
- /* Acquire mutex. Time spent holding mutex should be short! */
-#ifdef LWLOCK_STATS
- lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
-#else
- SpinLockAcquire(&lock->mutex);
-#endif
+ mustwait = pg_atomic_read_u32(&lock->lockcount) != 0;
- /* Is the lock now free, and if not, does the value match? */
- if (lock->exclusive == 0)
- {
- result = true;
- mustwait = false;
- }
- else
+ if (mustwait)
{
+ /*
+ * Perform comparison using spinlock as we can't rely on atomic 64
+ * bit reads/stores.
+ */
+ SpinLockAcquire(&lock->mutex);
+
value = *valptr;
if (value != oldval)
{
}
else
mustwait = true;
+ SpinLockRelease(&lock->mutex);
}
+ else
+ mustwait = false;
if (!mustwait)
break; /* the lock was free or value didn't match */
/*
- * Add myself to wait queue.
+ * Add myself to wait queue. Note that this is racy, somebody else
+ * could wakeup before we're finished queuing.
+ * NB: We're using nearly the same twice-in-a-row lock acquisition
+ * protocol as LWLockAcquire(). Check its comments for details.
*/
- proc->lwWaiting = true;
- proc->lwWaitMode = LW_WAIT_UNTIL_FREE;
- /* waiters are added to the front of the queue */
- dlist_push_head((dlist_head *) &lock->waiters, &proc->lwWaitLink);
+ LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);
- /* Can release the mutex now */
- SpinLockRelease(&lock->mutex);
+ /*
+ * We're now guaranteed to be woken up if necessary. Recheck the
+ * lock's state.
+ */
+ pg_read_barrier();
+ mustwait = pg_atomic_read_u32(&lock->lockcount) != 0;
+
+ /* ok, grabbed the lock the second time round, need to undo queueing */
+ if (!mustwait)
+ {
+#ifdef LWLOCK_STATS
+ lwstats->dequeue_self_count++;
+#endif
+ if (!LWLockDequeueSelf(lock))
+ {
+ /*
+ * Somebody else dequeued us and has or will wake us up. Wait
+ * for the correct wakeup, otherwise our ->lwWaiting would get
+ * reset at some inconvenient point later.
+ */
+ for (;;)
+ {
+ PGSemaphoreLock(&proc->sem, false);
+ if (!proc->lwWaiting)
+ break;
+ extraWaits++;
+ }
+ }
+ PRINT_LWDEBUG("LWLockWaitForVar undo queue", lock, LW_EXCLUSIVE);
+ break;
+ }
+ else
+ {
+ PRINT_LWDEBUG("LWLockWaitForVar waiting 4", lock, LW_EXCLUSIVE);
+ }
+
+ /*
+ * NB: Just as in LWLockAcquireCommon() there's no need to deal with
+ * spurious lock attempts here.
+ */
/*
* Wait until awakened.
LW_EXCLUSIVE);
LOG_LWDEBUG("LWLockWaitForVar", T_NAME(lock), T_ID(lock), "awakened");
+ pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);
/* Now loop back and check the status of the lock again. */
}
- /* We are done updating shared state of the lock itself. */
- SpinLockRelease(&lock->mutex);
-
TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), T_ID(lock), LW_EXCLUSIVE);
/*
/* Acquire mutex. Time spent holding mutex should be short! */
SpinLockAcquire(&lock->mutex);
- /* we should hold the lock */
- Assert(lock->exclusive == 1);
+ Assert(pg_atomic_read_u32(&lock->lockcount) >= EXCLUSIVE_LOCK);
/* Update the lock's value */
*valptr = val;
/*
* Awaken any waiters I removed from the queue.
*/
- dlist_foreach_modify(iter, (dlist_head *) &wakeup)
+ dlist_foreach_modify(iter, &wakeup)
{
PGPROC *waiter = dlist_container(PGPROC, lwWaitLink, iter.cur);
dlist_delete(&waiter->lwWaitLink);
void
LWLockRelease(LWLock *lock)
{
- dlist_head wakeup;
- dlist_mutable_iter iter;
+ LWLockMode mode;
+ uint32 lockcount;
+ bool check_waiters;
+ bool have_waiters = false;
int i;
- dlist_init(&wakeup);
-
- PRINT_LWDEBUG("LWLockRelease", lock);
-
/*
* Remove lock from list of locks held. Usually, but not always, it will
* be the latest-acquired lock; so search array backwards.
*/
for (i = num_held_lwlocks; --i >= 0;)
{
- if (lock == held_lwlocks[i])
+ if (lock == held_lwlocks[i].lock)
+ {
+ mode = held_lwlocks[i].mode;
break;
+ }
}
if (i < 0)
elog(ERROR, "lock %s %d is not held", T_NAME(lock), T_ID(lock));
for (; i < num_held_lwlocks; i++)
held_lwlocks[i] = held_lwlocks[i + 1];
- /* Acquire mutex. Time spent holding mutex should be short! */
- SpinLockAcquire(&lock->mutex);
+ PRINT_LWDEBUG("LWLockRelease", lock, mode);
- /* Release my hold on lock */
- if (lock->exclusive > 0)
- lock->exclusive--;
+ /* Release my hold on lock, both are a full barrier */
+ if (mode == LW_EXCLUSIVE)
+ lockcount = pg_atomic_sub_fetch_u32(&lock->lockcount, EXCLUSIVE_LOCK);
else
- {
- Assert(lock->shared > 0);
- lock->shared--;
- }
+ lockcount = pg_atomic_sub_fetch_u32(&lock->lockcount, 1);
+
+ /* nobody else can have that kind of lock */
+ Assert(lockcount < EXCLUSIVE_LOCK);
/*
- * See if I need to awaken any waiters. If I released a non-last shared
- * hold, there cannot be anything to do. Also, do not awaken any waiters
- * if someone has already awakened waiters that haven't yet acquired the
- * lock.
+ * Anybody we need to wakeup needs to have started queueing before
+ * we removed ourselves from the queue and the __sync_ operations
+ * above are full barriers.
*/
- if (lock->exclusive == 0 && lock->shared == 0 && lock->releaseOK)
- {
- /*
- * Remove the to-be-awakened PGPROCs from the queue.
- */
- bool releaseOK = true;
- bool wokeup_somebody = false;
- dlist_foreach_modify(iter, (dlist_head *) &lock->waiters)
- {
- PGPROC *waiter = dlist_container(PGPROC, lwWaitLink, iter.cur);
+ if (pg_atomic_read_u32(&lock->nwaiters) > 0)
+ have_waiters = true;
- if (wokeup_somebody && waiter->lwWaitMode == LW_EXCLUSIVE)
- continue;
-
- dlist_delete(&waiter->lwWaitLink);
- dlist_push_tail(&wakeup, &waiter->lwWaitLink);
-
- /*
- * Prevent additional wakeups until retryer gets to
- * run. Backends that are just waiting for the lock to become
- * free don't retry automatically.
- */
- if (waiter->lwWaitMode != LW_WAIT_UNTIL_FREE)
- {
- releaseOK = false;
- wokeup_somebody = true;
- }
+ /* grant permission to run, even if a spurious share lock increases lockcount */
+ if (mode == LW_EXCLUSIVE && have_waiters)
+ check_waiters = true;
+ /* nobody has this locked anymore, potential exclusive lockers get a chance */
+ else if (lockcount == 0 && have_waiters)
+ check_waiters = true;
+ /* nobody queued or not free */
+ else
+ check_waiters = false;
- if(waiter->lwWaitMode == LW_EXCLUSIVE)
- break;
- }
- lock->releaseOK = releaseOK;
+ if (check_waiters)
+ {
+ PRINT_LWDEBUG("LWLockRelease releasing", lock, mode);
+ LWLockWakeup(lock, mode);
}
- /* We are done updating shared state of the lock itself. */
- SpinLockRelease(&lock->mutex);
-
TRACE_POSTGRESQL_LWLOCK_RELEASE(T_NAME(lock), T_ID(lock));
- /*
- * Awaken any waiters I removed from the queue.
- */
- dlist_foreach_modify(iter, (dlist_head *) &wakeup)
- {
- PGPROC *waiter = dlist_container(PGPROC, lwWaitLink, iter.cur);
- LOG_LWDEBUG("LWLockRelease", T_NAME(l), T_ID(l), "release waiter");
- dlist_delete(&waiter->lwWaitLink);
- pg_write_barrier();
- waiter->lwWaiting = false;
- PGSemaphoreUnlock(&waiter->sem);
- }
-
/*
* Now okay to allow cancel/die interrupts.
*/
{
HOLD_INTERRUPTS(); /* match the upcoming RESUME_INTERRUPTS */
- LWLockRelease(held_lwlocks[num_held_lwlocks - 1]);
+ LWLockRelease(held_lwlocks[num_held_lwlocks - 1].lock);
}
}
/*
* LWLockHeldByMe - test whether my process currently holds a lock
*
- * This is meant as debug support only. We do not distinguish whether the
- * lock is held shared or exclusive.
+ * This is meant as debug support only. We currently do not distinguish
+ * whether the lock is held shared or exclusive.
*/
bool
LWLockHeldByMe(LWLock *l)
for (i = 0; i < num_held_lwlocks; i++)
{
- if (held_lwlocks[i] == l)
+ if (held_lwlocks[i].lock == l)
return true;
}
return false;
projects / users / andresfreund / postgres.git / commitdiff