From b13c5f2883af97cfb324a9884394b2e47b85a028 Mon Sep 17 00:00:00 2001 From: Attilio Rao Date: Wed, 23 Sep 2009 21:38:57 +0000 Subject: [PATCH] rwlock implemented from libthr need to fall through the 'hard path' and query umtx also if the shared waiters bit is set on a shared lock. The writer starvation avoidance technique, infact, can lead to shared waiters on a shared lock which can bring to a missed wakeup and thus to a deadlock if the right bit is not checked (a notable case is the writers counterpart to be handled through expired timeouts). Fix that by checking for the shared waiters bit also when unlocking the shared locks. That bug was causing a reported MySQL deadlock. Many thanks go to Nick Esborn and his employer DesertNet which provided time and machines to identify and fix this issue. PR: thread/135673 Reported by: Nick Esborn Tested by: Nick Esborn Reviewed by: jeff --- lib/libthr/thread/thr_umtx.h | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/lib/libthr/thread/thr_umtx.h b/lib/libthr/thread/thr_umtx.h index 41b5f9686783..a6e462e29ca5 100644 --- a/lib/libthr/thread/thr_umtx.h +++ b/lib/libthr/thread/thr_umtx.h @@ -171,8 +171,11 @@ _thr_rwlock_unlock(struct urwlock *rwlock) for (;;) { if (__predict_false(URWLOCK_READER_COUNT(state) == 0)) return (EPERM); - if (!((state & URWLOCK_WRITE_WAITERS) && URWLOCK_READER_COUNT(state) == 1)) { - if (atomic_cmpset_rel_32(&rwlock->rw_state, state, state-1)) + if (!((state & (URWLOCK_WRITE_WAITERS | + URWLOCK_READ_WAITERS)) && + URWLOCK_READER_COUNT(state) == 1)) { + if (atomic_cmpset_rel_32(&rwlock->rw_state, + state, state-1)) return (0); state = rwlock->rw_state; } else {