Prevent potentially hazardous compiler/cpu reordering during lwlock release.

In LWLockRelease() (and in 9.4+ LWLockUpdateVar()) we release enqueued
waiters using PGSemaphoreUnlock(). As there are other sources of such
unlocks backends only wake up if MyProc->lwWaiting is set to false;
which is only done in the aforementioned functions.

Before this commit there were dangers because the store to lwWaitLink
could become visible before the store to lwWaitLink. This could both
happen due to compiler reordering (on most compilers) and on some
platforms due to the CPU reordering stores.

The possible consequence of this is that a backend stops waiting
before lwWaitLink is set to NULL. If that backend then tries to
acquire another lock and has to wait there the list could become
corrupted once the lwWaitLink store is finally performed.

Add a write memory barrier to prevent that issue.

Unfortunately the barrier support has been only added in 9.2. Given
that the issue has not knowingly been observed in praxis it seems
sufficient to prohibit compiler reordering using volatile for 9.0 and
9.1. Actual problems due to compiler reordering are more likely
anyway.

Discussion: 20140210134625.GA15246@awork2.anarazel.de

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index f482f34bad3bd9402669d2f1f03aa178e0abe2e4..61eadeff6b4f0e07e3882f7ca1f400f4d9e9d605 100644 (file)
--- 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);
        }
 
        /*