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Message-ID: <01b301c0fa5e$5ecae940$0200a8c0@lifelesswks>
From: "Robert Collins" <robert DOT collins AT itdomain DOT com DOT au>
To: "Greg Smith" <gsmith AT nc DOT rr DOT com>, <cygwin AT cygwin DOT com>
References: <3B3181E5 DOT E6F0D06A AT nc DOT rr DOT com>
Subject: Re: Deadly embrace between pthread_cond_wait and pthread_cond_signal
Date: Fri, 22 Jun 2001 00:27:58 +1000
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X-OriginalArrivalTime: 21 Jun 2001 14:16:36.0232 (UTC) FILETIME=[C7CBA080:01C0FA5C]

Actually, I think your solution here is the right one... I'm sending in
a patch based on it.

Rob
----- Original Message -----
From: "Greg Smith" <gsmith AT nc DOT rr DOT com>
To: <cygwin AT cygwin DOT com>
Sent: Thursday, June 21, 2001 3:11 PM
Subject: Deadly embrace between pthread_cond_wait and
pthread_cond_signal


> I am using the cygwin-src snapshot from June 19.
>
> Suppose I have a thread, t1, that looks like:
> void *t1() {
>    pthread_mutex_lock(&lock);
>    pthread_cond_wait(&cond,&lock);
>    pthread_mutex_unlock(&lock);
> }
> and a thread, t2, that looks like:
> void *t2() {
>    pthread_mutex_lock(&lock);
>    pthread_cond_signal(&cond);
>    sleep(1);
>    pthread_cond_signal(&cond);
>    pthread_mutex_unlock(&lock);
> }
>
> When thread t1 wakes up as a result of the first pthread_cond_signal
> issued by thread t2, __pthread_cond_wait in thread.cc obtains internal
> mutex `cond_access' then hangs trying to acquire external mutex `lock'
> which is owned by thread t2.
>
> When thread t2 issues the second pthread_cond_signal, still holding
> external mutex `lock', pthread_cond::Signal in thread.cc tries to
> get internal mutex `cond_access' and hangs because it is owned by t1.
>
> So, t1 has `cond_access' and is waiting on `lock'
> and t2 has `lock' and is waiting on `cond_access'.
>
> As a workaround, I moved the pthread_mutex_lock for cond_access in
> __pthread_cond_wait from before the (*cond)->mutex->Lock() to after
> it, and my application has gotten a whole lot further than ever before
> using native cygwin pthreads, although it does seem to be running as
> slow as molasses compared to linux.  But maybe that's cause of my
> debug stuff I had to add.
>
> Thanks,
>
> Greg
>
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>


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