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From: "mihaylov.mihail at gmail dot com" <sourceware-bugzilla@sourceware.org> To: glibc-bugs@sources.redhat.com Subject: [Bug nptl/13165] pthread_cond_wait() can consume a signal that was sent before it started waiting Date: Wed, 28 Sep 2011 09:03:00 -0000 [thread overview] Message-ID: <bug-13165-131-46WcqILMoF@http.sourceware.org/bugzilla/> (raw) In-Reply-To: <bug-13165-131@http.sourceware.org/bugzilla/> http://sourceware.org/bugzilla/show_bug.cgi?id=13165 --- Comment #13 from Mihail Mihaylov <mihaylov.mihail at gmail dot com> 2011-09-28 09:02:51 UTC --- (In reply to comment #11) > I've confirmed that the issue occurs on my Debian system with their libc6 > package (eglibc 2.13-10, albeit slightly different from glibc). I originally observed the problem on a Debian stable. I've run my test case on my laptop which is running Mint and on my office workstation which is running kubuntu. I looked at the eglibc source code before posting the bug and saw that the code which causes the race is identical to the one in glibc, so the bug is in both implementations. > I've also confirmed that the issue does not occur with my implementation of > condition variables in musl libc(*). I took a look at your code. As far as I can tell, you are not trying to avoid spurious wakeups as hard as glibc, that's why you don't have the same race. > I suspect it's a real bug, but I need to read the code more closely to > understand what's going on... Here is my understanding of the root cause - an attempt to prevent spurious wakeups that has gone too far and destroys ordering - waking future waiters instead of present ones. There are two checks that NPTL uses to prevent spurious wakeups: 1) It only allows a thread to wake if a signal has been sent after it started waiting. This is achieved by checking if cond->__data.__wakeup_seq has remained unchanged. 2) It only allows as many threads to wake up as there were signals. This is achieved by checking if cond->__data._woken_seq equals cond->__data.__wakeup_seq. If any of this checks indicates a spurious wakeup the thread retries the wait. The problem is in check 2, because the guard is triggered if any thread has woken spuriously - not just the current thread. Worse - it is triggered only after the spuriously woken thread consumed a signal. So in many cases the spuriously woken thread consumes the signal, and a validly woken thread is forced to retry. The result is that a spurious wakeup may steal signals that were sent before it started waiting. Now, I'm confident that the race is real. But maybe some people would disagree that it is a bug. That's why I asked in my original message if this behaviour is intentional or a bug. It is a bug if pthread condition variables should support the following usage: ... pthread_mutex_lock(&m); SomeType localState = f(sharedState); while ( predicate(sharedState, localState) ) { pthread_cond_wait(&c, &m); } ... In this case it actually matters which thread will wake up, because if the wrong thread wakes up, it will retry the wait and the signal will be lost (this is what happened to me). Unfortunately the spec is not very clear on the issue. But this is the pattern that the pthread_cond_wait implementation in glibc itself uses to detect spurious wakeups on the futex. -- Configure bugmail: http://sourceware.org/bugzilla/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- You are on the CC list for the bug.
next prev parent reply other threads:[~2011-09-28 9:03 UTC|newest] Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-09-07 19:15 [Bug nptl/13165] New: " mihaylov.mihail at gmail dot com 2011-09-21 9:12 ` [Bug nptl/13165] " mihaylov.mihail at gmail dot com 2011-09-21 18:19 ` bugdal at aerifal dot cx 2011-09-21 22:29 ` bugdal at aerifal dot cx 2011-09-22 22:21 ` mihaylov.mihail at gmail dot com 2011-09-25 21:33 ` mihaylov.mihail at gmail dot com 2011-09-25 21:44 ` mihaylov.mihail at gmail dot com 2011-09-26 9:27 ` mihaylov.mihail at gmail dot com 2011-09-26 16:20 ` bugdal at aerifal dot cx 2011-09-27 10:10 ` mihaylov.mihail at gmail dot com 2011-09-27 10:13 ` mihaylov.mihail at gmail dot com 2011-09-28 2:07 ` bugdal at aerifal dot cx 2011-09-28 2:08 ` bugdal at aerifal dot cx 2011-09-28 9:03 ` mihaylov.mihail at gmail dot com [this message] 2011-09-28 16:06 ` bugdal at aerifal dot cx 2011-09-28 21:00 ` mihaylov.mihail at gmail dot com 2012-09-19 15:15 ` triegel at redhat dot com 2012-09-19 15:21 ` triegel at redhat dot com 2012-09-19 17:23 ` bugdal at aerifal dot cx 2012-09-20 10:28 ` mihaylov.mihail at gmail dot com 2012-09-20 10:43 ` triegel at redhat dot com 2012-09-20 11:05 ` mihaylov.mihail at gmail dot com 2012-09-20 11:23 ` triegel at redhat dot com 2012-09-20 11:58 ` triegel at redhat dot com 2012-09-20 12:46 ` mihaylov.mihail at gmail dot com 2012-09-20 12:49 ` mihaylov.mihail at gmail dot com 2012-09-20 16:21 ` triegel at redhat dot com 2012-09-20 18:39 ` bugdal at aerifal dot cx 2012-09-20 19:48 ` mihaylov.mihail at gmail dot com 2012-09-20 20:31 ` bugdal at aerifal dot cx 2012-09-21 8:04 ` triegel at redhat dot com 2012-09-21 8:05 ` siddhesh at redhat dot com 2012-09-21 8:54 ` bugdal at aerifal dot cx 2012-09-21 15:45 ` triegel at redhat dot com 2012-10-18 6:26 ` mihaylov.mihail at gmail dot com 2012-10-18 12:25 ` bugdal at aerifal dot cx 2012-10-24 20:26 ` triegel at redhat dot com 2012-10-25 4:08 ` bugdal at aerifal dot cx 2013-01-19 16:19 ` scot4spam at yahoo dot com 2014-02-16 17:45 ` jackie.rosen at hushmail dot com 2014-03-28 9:23 ` dancol at dancol dot org 2014-05-28 19:44 ` schwab at sourceware dot org 2014-06-27 12:09 ` fweimer at redhat dot com 2014-08-18 21:22 ` triegel at redhat dot com 2014-08-18 21:42 ` bugdal at aerifal dot cx 2015-08-26 15:29 ` kkersten at cray dot com 2017-01-01 21:32 ` triegel at redhat dot com
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