Re: A Bug in pthread_cond_destroy?
Re: A Bug in pthread_cond_destroy?
- Subject: Re: A Bug in pthread_cond_destroy?
- From: "Kazuho Oku" <email@hidden>
- Date: Thu, 10 Jan 2008 22:09:01 +0900
2008/1/10, Jim Magee <email@hidden>:
> Yes, I saw the quote about it being "safe to destroy a mutex on which
> no thread is waiting" but the fact that the sentence and paragraph
> before said that attempting to use such a destroyed condition would
> result in undefined behavior still holds.
Well, my example never calls pthread_cond_wait AFTER calling
pthread_cond_destroy. Maybe I was vague on my explatation, what
happens is:
1) thread-A enters pthread_cond_wait
2) thread-B enters pthread_cond_signal
3) thread-A returns from pthread_cond_wait
4) thread-A calls pthread_cond_destroy
5) thread-A returns from pthread_cond_destroy (returns 0)
6) thread-B read & writes the conditional
7) thread-B returns from pthread_cond_signal
which from my understanding is the exact case the spec describes as
"it shall be safe to destroy an initialized condition variable upon
which no threads are currently blocked."
Or if what you are trying to say is that the above sequence is valid
by the pthread specification, sorry for bothering your time.
> In your case, the destroy _was_ done safely (the destroy didn't hang
> or have any other odd behavior, correct?). It was the attempt to use
> the destroyed condition variable that caused problems - and that is
> exactly what the spec says is undefined. So, I think our
> implementation meets the spec.
>
> --Jim
>
> On Jan 9, 2008, at 8:01 AM, Kazuho Oku wrote:
>
> > 2008/1/9, Jim Magee <email@hidden>:
> >
> >> I think you are assuming behavior that isn't specified.
> >> Specifically, the
> >> definition for pthread_cond_destroy() says:
> >> The pthread_cond_destroy() function shall destroy the given condition
> >> variable specified by cond; the object becomes, in effect,
> >> uninitialized. An
> >> implementation may cause pthread_cond_destroy() to set the object
> >> referenced
> >> by cond to an invalid value. A destroyed condition variable object
> >> can be
> >> reinitialized using pthread_cond_init(); the results of otherwise
> >> referencing the object after it has been destroyed are undefined.
> >
> > Thank you for your response, I did not know such a statement exists in
> > pthread spec., maybe I was referring to an older document.
> >
> > Thas said, I googled and found a spec. that includes your quotes
> > (http://www.opengroup.org/onlinepubs/007908775/xsh/pthread_cond_destroy.html
> > ),
> > and if you are referrring to it, my understaning is that it states
> >
> > It shall be safe to destroy an initialized condition variable upon
> > which no threads are currently blocked.
> >
> > which is the case in my example. My understanding is that their is
> > the case where a waiting thread get signalled and woke up and may
> > destroy the cond before the signalling thread finishes modifying the
> > pthread_cond_t variable.
> >
> > Or if there's any other version, would you please let me know?
> >
> > Thank you in advance.
> >
> >> So, trying to call pthread_cond_signal() from another thread after
> >> the
> >> condition is destroyed (or even "during" it being destroyed) can
> >> result is
> >> undefined behavior. You really need synchronization of your own to
> >> avoid
> >> issues there. Using the same mutex would have been good enough.
> >> Since that
> >> isn't in your model, you need something else (maybe a second
> >> condition
> >> variable that indicates whether the first has been destroyed? - just
> >> thinking out loud).
> >>
> >> --Jim
> >>
> >>
> >> On Jan 9, 2008, at 2:25 AM, Kazuho Oku wrote:
> >>
> >> Hi,
> >>
> >> If this is not the right place to report bugs of libc, please tell me
> >> the right mailing list. Thank you in advance, and sorry, if it is
> >> the
> >> case.
> >>
> >> I think I have found a bug in pthread_cond_destroy of Libc (with Mac
> >> OS X 10.4.10).
> >>
> >> Current implementation of pthread_cond_destroy does not check if
> >> sigspending==0. Thus if pthread_cond_destroy is called from a thread
> >> other than that called pthread_cond_signal, the destructor may return
> >> 0 even if pthread_cond_signal is still in work (i.e. sigspending !=
> >> 0), leading to memory corruption or an infinite loop in
> >> pthread_cond_signal.
> >>
> >> For example, in the following code, thread B would sometimes read
> >> from
> >> and/or write to freed memory.
> >>
> >> Thread A:
> >> pthread_cond_wait(cond, &mutex);
> >> while ((err = pthread_cond_destroy(cond)) != 0) {
> >> assert(err == EBUSY);
> >> usleep(1);
> >> }
> >> free(cond);
> >>
> >> Thread B:
> >> pthread_cond_signal(global_cond);
> >>
> >> Note that the problem only arises when thread B does not lock the
> >> same
> >> mutex as thread A when calling thread A (this is not a requirement in
> >> POSIX threads).
> >>
> >> Attached to this mail is a code that would reproduce this bug. It
> >> sends and receives pthread_cond_signals and increment a counter.
> >> It compiled without any compile time defines, the program prints the
> >> counter incrementing infinitely.
> >> But if compiled with -DCLEAR_MEM -DSIGNAL_WO_LOCK it would suddenly
> >> stop due to memory corruption.
> >> Or if compiled with -DAPPLE_TEST -DSIGNAL_WO_LOCK it would check the
> >> value of sispending and print an assertion failure.
> >>
> >> To reproduce the bug with the attached code, a dual processor system
> >> might be a requirement.
> >>
> >> --
> >> Kazuho Oku
> >> <thtest.c> _______________________________________________
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> >
> >
> > --
> > Kazuho Oku
>
>
--
Kazuho Oku
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