Re: Deadly embrace between pthread_cond_wait and pthread_cond_signal

["Robert Collins" <robert dot collins at itdomain dot com dot au>]

Correct a deadlock situation...
Changelog:
2001-06-22  Robert Collins  rbtcollins@hotmail.com

    * thread.cc (__pthread_cond_timedwait): Lock the waiting mutex
before the condition protect mutex to avoid deadlocking.
    (__pthread_cond_wait): Ditto.


Greg, as far as speed goes, there is a major optimisation I want to
make, which is to make all process private mutex's critical sections,
instead of system wide mutex's. This should be a lot faster... when I
get that done :].

Rob

----- Original Message -----
From: "Greg Smith" <gsmith@nc.rr.com>
To: <cygwin@cygwin.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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deadlockwait.patch