diff options
| -rw-r--r-- | src/backend/storage/lmgr/lwlock.c | 20 |
1 files changed, 15 insertions, 5 deletions
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c index f482f34bad3..61eadeff6b4 100644 --- a/src/backend/storage/lmgr/lwlock.c +++ b/src/backend/storage/lmgr/lwlock.c @@ -647,12 +647,22 @@ LWLockRelease(LWLockId lockid) */ while (head != NULL) { + /* + * Try to guarantee that lwWaiting being unset only becomes visible + * once the unlink from the link has completed. Otherwise the target + * backend could be woken up for other reason and enqueue for a new + * lock - if that happens before the list unlink happens, the list + * would end up being corrupted. In later releases we can rely on + * barriers, but < 9.2 doesn't yet have them - so just use volatile. + */ + volatile PGPROC *p; + LOG_LWDEBUG("LWLockRelease", lockid, "release waiter"); - proc = head; - head = proc->lwWaitLink; - proc->lwWaitLink = NULL; - proc->lwWaiting = false; - PGSemaphoreUnlock(&proc->sem); + p = head; + head = p->lwWaitLink; + p->lwWaitLink = NULL; + p->lwWaiting = false; + PGSemaphoreUnlock((PGSemaphore) &p->sem); } /* |