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

[Jens Gustedt <jens.gustedt@inria.fr>]

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Am Mittwoch, den 13.08.2014, 08:43 +0200 schrieb Jens Gustedt:
> Am Dienstag, den 12.08.2014, 17:18 -0400 schrieb Rich Felker:
> > On Tue, Aug 12, 2014 at 09:09:21PM +0200, Jens Gustedt wrote:
> > > Rich,
> > > thanks a lot for looking into the code.
> > > 
> > > Am Dienstag, den 12.08.2014, 12:01 -0400 schrieb Rich Felker:
> > > > As far as I can tell, the only thing that's saving you from sending
> > > > futex wakes after free is that you're just using spinlocks.
> > > 
> > > No, I don't think so. These protect critical sections at the beginning
> > > of the cnd_t calls. The cnd_*wait calls hold the mutex at that time
> > > anyhow, so even if these would be implemented with mutexes (an extra
> > > one per cnd_t to protect the critical section) this wouldn't cause
> > > late wakes, I think.
> > 
> > I was talking about the unref-and-free code that's using spinlocks.
> 
> I don't understand, the unref-and-free code doesn't use spinlocks. It
> just uses an atomic_fetch_sub of 1 to determine if it is the last
> user. If it is not, it does nothing and returns.
> 
> (This is indeed supposing that atomic_fetch_sub is "lock-free" in the
> sense of the C standard, which basically means that it has no
> observable state from a signal handler. All architectures I know of
> have that property, but my knowledge is limited.)

ah, perhaps you meant that the unref-and-free was done while holding
the spinlock? Maybe in the version that I posted this was the
case. That was a bug that I have now corrected.

I also have added an initial check for the wakers to see if they have
anything to do before they go into the spinlock. This should basically
solve the herd-doing-wakeup problem, since as soon as all waiters are
considered to be unblocked, wakers will return immediately without
spinning.

Jens

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