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-rw-r--r--src/backend/access/transam/twophase.c2
-rw-r--r--src/backend/storage/lmgr/lwlock.c53
-rw-r--r--src/backend/storage/lmgr/proc.c4
-rw-r--r--src/include/storage/lwlock.h8
-rw-r--r--src/include/storage/proc.h2
5 files changed, 42 insertions, 27 deletions
diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 076315e7a28..db68b144431 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -482,7 +482,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
proc->roleId = owner;
proc->tempNamespaceId = InvalidOid;
proc->isBackgroundWorker = false;
- proc->lwWaiting = false;
+ proc->lwWaiting = LW_WS_NOT_WAITING;
proc->lwWaitMode = 0;
proc->waitLock = NULL;
proc->waitProcLock = NULL;
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index d63dc1b93ea..28a540961ea 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -994,6 +994,15 @@ LWLockWakeup(LWLock *lock)
}
/*
+ * Signal that the process isn't on the wait list anymore. This allows
+ * LWLockDequeueSelf() to remove itself of the waitlist with a
+ * proclist_delete(), rather than having to check if it has been
+ * removed from the list.
+ */
+ Assert(waiter->lwWaiting == LW_WS_WAITING);
+ waiter->lwWaiting = LW_WS_PENDING_WAKEUP;
+
+ /*
* Once we've woken up an exclusive lock, there's no point in waking
* up anybody else.
*/
@@ -1050,7 +1059,7 @@ LWLockWakeup(LWLock *lock)
* another lock.
*/
pg_write_barrier();
- waiter->lwWaiting = false;
+ waiter->lwWaiting = LW_WS_NOT_WAITING;
PGSemaphoreUnlock(waiter->sem);
}
}
@@ -1071,7 +1080,7 @@ LWLockQueueSelf(LWLock *lock, LWLockMode mode)
if (MyProc == NULL)
elog(PANIC, "cannot wait without a PGPROC structure");
- if (MyProc->lwWaiting)
+ if (MyProc->lwWaiting != LW_WS_NOT_WAITING)
elog(PANIC, "queueing for lock while waiting on another one");
LWLockWaitListLock(lock);
@@ -1079,7 +1088,7 @@ LWLockQueueSelf(LWLock *lock, LWLockMode mode)
/* setting the flag is protected by the spinlock */
pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_HAS_WAITERS);
- MyProc->lwWaiting = true;
+ MyProc->lwWaiting = LW_WS_WAITING;
MyProc->lwWaitMode = mode;
/* LW_WAIT_UNTIL_FREE waiters are always at the front of the queue */
@@ -1107,8 +1116,7 @@ LWLockQueueSelf(LWLock *lock, LWLockMode mode)
static void
LWLockDequeueSelf(LWLock *lock)
{
- bool found = false;
- proclist_mutable_iter iter;
+ bool on_waitlist;
#ifdef LWLOCK_STATS
lwlock_stats *lwstats;
@@ -1121,18 +1129,13 @@ LWLockDequeueSelf(LWLock *lock)
LWLockWaitListLock(lock);
/*
- * Can't just remove ourselves from the list, but we need to iterate over
- * all entries as somebody else could have dequeued us.
+ * Remove ourselves from the waitlist, unless we've already been
+ * removed. The removal happens with the wait list lock held, so there's
+ * no race in this check.
*/
- proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
- {
- if (iter.cur == MyProc->pgprocno)
- {
- found = true;
- proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
- break;
- }
- }
+ on_waitlist = MyProc->lwWaiting == LW_WS_WAITING;
+ if (on_waitlist)
+ proclist_delete(&lock->waiters, MyProc->pgprocno, lwWaitLink);
if (proclist_is_empty(&lock->waiters) &&
(pg_atomic_read_u32(&lock->state) & LW_FLAG_HAS_WAITERS) != 0)
@@ -1144,8 +1147,8 @@ LWLockDequeueSelf(LWLock *lock)
LWLockWaitListUnlock(lock);
/* clear waiting state again, nice for debugging */
- if (found)
- MyProc->lwWaiting = false;
+ if (on_waitlist)
+ MyProc->lwWaiting = LW_WS_NOT_WAITING;
else
{
int extraWaits = 0;
@@ -1169,7 +1172,7 @@ LWLockDequeueSelf(LWLock *lock)
for (;;)
{
PGSemaphoreLock(MyProc->sem);
- if (!MyProc->lwWaiting)
+ if (MyProc->lwWaiting == LW_WS_NOT_WAITING)
break;
extraWaits++;
}
@@ -1320,7 +1323,7 @@ LWLockAcquire(LWLock *lock, LWLockMode mode)
for (;;)
{
PGSemaphoreLock(proc->sem);
- if (!proc->lwWaiting)
+ if (proc->lwWaiting == LW_WS_NOT_WAITING)
break;
extraWaits++;
}
@@ -1485,7 +1488,7 @@ LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
for (;;)
{
PGSemaphoreLock(proc->sem);
- if (!proc->lwWaiting)
+ if (proc->lwWaiting == LW_WS_NOT_WAITING)
break;
extraWaits++;
}
@@ -1701,7 +1704,7 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
for (;;)
{
PGSemaphoreLock(proc->sem);
- if (!proc->lwWaiting)
+ if (proc->lwWaiting == LW_WS_NOT_WAITING)
break;
extraWaits++;
}
@@ -1779,6 +1782,10 @@ LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 val)
proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
proclist_push_tail(&wakeup, iter.cur, lwWaitLink);
+
+ /* see LWLockWakeup() */
+ Assert(waiter->lwWaiting == LW_WS_WAITING);
+ waiter->lwWaiting = LW_WS_PENDING_WAKEUP;
}
/* We are done updating shared state of the lock itself. */
@@ -1794,7 +1801,7 @@ LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 val)
proclist_delete(&wakeup, iter.cur, lwWaitLink);
/* check comment in LWLockWakeup() about this barrier */
pg_write_barrier();
- waiter->lwWaiting = false;
+ waiter->lwWaiting = LW_WS_NOT_WAITING;
PGSemaphoreUnlock(waiter->sem);
}
}
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1e06d6580ce..f62e622a7bf 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -400,7 +400,7 @@ InitProcess(void)
/* NB -- autovac launcher intentionally does not set IS_AUTOVACUUM */
if (IsAutoVacuumWorkerProcess())
MyProc->statusFlags |= PROC_IS_AUTOVACUUM;
- MyProc->lwWaiting = false;
+ MyProc->lwWaiting = LW_WS_NOT_WAITING;
MyProc->lwWaitMode = 0;
MyProc->waitLock = NULL;
MyProc->waitProcLock = NULL;
@@ -583,7 +583,7 @@ InitAuxiliaryProcess(void)
MyProc->delayChkpt = false;
MyProc->delayChkptEnd = false;
MyProc->statusFlags = 0;
- MyProc->lwWaiting = false;
+ MyProc->lwWaiting = LW_WS_NOT_WAITING;
MyProc->lwWaitMode = 0;
MyProc->waitLock = NULL;
MyProc->waitProcLock = NULL;
diff --git a/src/include/storage/lwlock.h b/src/include/storage/lwlock.h
index 78afe57fb2a..b2c9a9dbbcc 100644
--- a/src/include/storage/lwlock.h
+++ b/src/include/storage/lwlock.h
@@ -23,6 +23,14 @@
struct PGPROC;
+/* what state of the wait process is a backend in */
+typedef enum LWLockWaitState
+{
+ LW_WS_NOT_WAITING, /* not currently waiting / woken up */
+ LW_WS_WAITING, /* currently waiting */
+ LW_WS_PENDING_WAKEUP /* removed from waitlist, but not yet signalled */
+} LWLockWaitState;
+
/*
* Code outside of lwlock.c should not manipulate the contents of this
* structure directly, but we have to declare it here to allow LWLocks to be
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 1464fad9b94..9cc37aa0d93 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -180,7 +180,7 @@ struct PGPROC
bool recoveryConflictPending;
/* Info about LWLock the process is currently waiting for, if any. */
- bool lwWaiting; /* true if waiting for an LW lock */
+ uint8 lwWaiting; /* see LWLockWaitState */
uint8 lwWaitMode; /* lwlock mode being waited for */
proclist_node lwWaitLink; /* position in LW lock wait list */
generated by cgit v1.2.3 (git 2.46.0) at 2025-09-09 02:47:45 +0000