Re: Explaining cond var destroy [Re: [musl] C threads, v3.0]

[Rich Felker <dalias@libc.org>]

On Thu, Aug 07, 2014 at 12:43:35AM +0200, Jens Gustedt wrote:
> Am Mittwoch, den 06.08.2014, 18:04 -0400 schrieb Rich Felker:
> > No. The problem is that the memory in which the cond var resides could
> > previously have been used for a mutex, semaphore, or other object for
> > which a futex wait arrives "very late" due to scheduling. It has
> > nothing to do with the destruction of cond vars.
> 
> Hm, but then we have a problem on the side that has this late wait,
> that is an access to a dead object. So the UB is there, in that thread
> that is doing the wait. (Suppose it is just threads and not processes,
> that would even be more complicated.) That spurious wakeup that you
> are talking about is a consequence of UB.

No, it's not. The wait happens prior to the deallocation, in the same
thread that performs the deallocation. The interleaving looks like
this:

Thread A                        Thread B
--------                        --------
waiters++
                                save waiters count
                                atomic unlock
futex wait fails with EAGAIN
cas succeeds & gets lock
waiters--
[unlock operation]
[free operation]
                                futex wake to freed address

The free operation in thread A is valid since A knows it is the last
user of the mutex and thread B's use/ownership of the mutex formally
ends with the atomic unlock.

Rich