[Bug nptl/12875] pthread_cond_timedwait can steal the wakeup of slower thread in pthread_cond_wait

["martin at lispworks dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=12875

martin at lispworks dot com changed:

           What    |Removed                     |Added
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             Status|RESOLVED                    |REOPENED
         Resolution|INVALID                     |

--- Comment #2 from martin at lispworks dot com 2011-06-13 12:39:04 UTC ---
Can you take another look at this please?  It isn't about fairness because the
man page (and spec AFAIK) for pthread_cond_signal says:

"The pthread_cond_signal() function shall unblock at least one of the threads
that are blocked on the specified condition variable cond (if any threads are
blocked on cond)."

In the example, only thread A is blocked at the time thread B signals for the
first time, so I think it should wake whatever happens later  The blocking of
threads C and D occurs after thread B signals for the first time so shouldn't
be affected by that signal.

The problem is that if pthread_cond_timedwait in thread D reaches its timeout
before thread A has had a chance to increment __woken_seq, then thread D will
claim the signal even though its true reason for waking is the timeout.

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