threads/135673: commit references a PR
dfilter service
dfilter at FreeBSD.ORG
Wed Sep 23 21:40:03 UTC 2009
The following reply was made to PR threads/135673; it has been noted by GNATS.
From: dfilter at FreeBSD.ORG (dfilter service)
To: bug-followup at 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 at freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/svn-src-all
To unsubscribe, send any mail to "svn-src-all-unsubscribe at freebsd.org"
More information about the freebsd-threads
mailing list