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rwlock implemented from libthr need to fall through the 'hard path' and

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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 <nick at desert dot net>
Tested by:	Nick Esborn <nick at desert dot net>
Reviewed by:	jeff
This commit is contained in:
Attilio Rao 2009-09-23 21:38:57 +00:00
parent 19de5df5e5
commit b13c5f2883
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=197445
1 changed files with 5 additions and 2 deletions
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7
lib/libthr/thread/thr_umtx.h
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@ -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 {