Date: Wed, 23 Sep 2009 21:40:02 GMT From: dfilter@FreeBSD.ORG (dfilter service) To: freebsd-threads@FreeBSD.org Subject: Re: threads/135673: commit references a PR Message-ID: <200909232140.n8NLe26N069252@freefall.freebsd.org>
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The following reply was made to PR threads/135673; it has been noted by GNATS. From: dfilter@FreeBSD.ORG (dfilter service) To: bug-followup@FreeBSD.org Cc: Subject: Re: threads/135673: commit references a PR Date: Wed, 23 Sep 2009 21:39:10 +0000 (UTC) Author: attilio Date: Wed Sep 23 21:38:57 2009 New Revision: 197445 URL: http://svn.freebsd.org/changeset/base/197445 Log: 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 <nick at desert dot net> Tested by: Nick Esborn <nick at desert dot net> Reviewed by: jeff Modified: head/lib/libthr/thread/thr_umtx.h Modified: head/lib/libthr/thread/thr_umtx.h ============================================================================== --- head/lib/libthr/thread/thr_umtx.h Wed Sep 23 20:49:14 2009 (r197444) +++ head/lib/libthr/thread/thr_umtx.h Wed Sep 23 21:38:57 2009 (r197445) @@ -171,8 +171,11 @@ _thr_rwlock_unlock(struct urwlock *rwloc 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 { _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscribe@freebsd.org"
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