Fix deadlock with LWLockAcquireWithVar and LWLockWaitForVar.

LWLockRelease should release all backends waiting with LWLockWaitForVar,
even when another backend has already been woken up to acquire the lock,
i.e. when releaseOK is false. LWLockWaitForVar can return as soon as the
protected value changes, even if the other backend will acquire the lock.
Fix that by resetting releaseOK to true in LWLockWaitForVar, whenever
adding itself to the wait queue.

This should fix the bug reported by MauMau, where the system occasionally
hangs when there is a lot of concurrent WAL activity and a checkpoint.
Backpatch to 9.4, where this code was added.

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index 5453549a792a2e4bd53cf207a6010d279ea1224b..1607bc9e536d3f66cdecc61ed31389ebbb11cca4 100644 (file)
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -976,6 +976,12 @@ LWLockWaitForVar(LWLock *l, uint64 *valptr, uint64 oldval, uint64 *newval)
                        lock->tail = proc;
                lock->head = proc;
 
+               /*
+                * Set releaseOK, to make sure we get woken up as soon as the lock is
+                * released.
+                */
+               lock->releaseOK = true;
+
                /* Can release the mutex now */
                SpinLockRelease(&lock->mutex);