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Subject: Re: [OMPI devel] opal_condition_wait
From: Gleb Natapov (glebn_at_[hidden])
Date: 2007-12-06 13:48:49

On Thu, Dec 06, 2007 at 09:46:45AM -0500, Tim Prins wrote:
> Also, when we are using threads, there is a case where we do not
> decrement the signaled count, in condition.h:84. Gleb put this in in
> r9451, however the change does not make sense to me. I think that the
> signal count should always be decremented.
> Can anyone shine any light on these issues?
I made this change a long time ago (I wander why I even tested threaded
build back then), but what I recall looking into the code and log message
there was a deadlock when signal broadcast doesn't wake up all thread
that are waiting on a conditional variable. Suppose two threads wait on
a condition C, third thread does broadcast. This makes C->c_signaled to
be equal 2. Now one thread wakes up and decrement C->c_signaled by one.
And before other thread is starting to run it calls condition_wait on C
one more time. Because c_signaled is 1 it doesn't sleep and decrement
c_signaled one more time. Now c_signaled is zero and when second thread
wakes up it see this and go to sleep again. The solution was to check in
condition_wait if condition is already signaled before go to sleep and
if yes exit immediately.