Date: Fri, 13 May 2011 13:13:11 -0400 From: Max Laier <max@love2party.net> To: freebsd-current@freebsd.org Cc: Andriy Gapon <avg@freebsd.org> Subject: Re: proposed smp_rendezvous change Message-ID: <201105131313.11677.max@love2party.net> In-Reply-To: <201105131150.57548.max@love2party.net> References: <4DCD357D.6000109@FreeBSD.org> <4DCD4E21.7020800@FreeBSD.org> <201105131150.57548.max@love2party.net>
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On Friday 13 May 2011 11:50:57 Max Laier wrote:
> On Friday 13 May 2011 11:28:33 Andriy Gapon wrote:
> > on 13/05/2011 17:41 Max Laier said the following:
> > > this ncpus isn't the one you are looking for.
> >
> > Thank you!
>
> > Here's an updated patch:
> Can you attach the patch, so I can apply it locally. This code is really
> hard to read without context. Some more comments inline ...
>
> > Index: sys/kern/subr_smp.c
> > ===================================================================
> > --- sys/kern/subr_smp.c (revision 221835)
> > +++ sys/kern/subr_smp.c (working copy)
> > @@ -316,19 +316,14 @@
> >
> > void (*local_action_func)(void*) = smp_rv_action_func;
> > void (*local_teardown_func)(void*) = smp_rv_teardown_func;
> >
> > - /* Ensure we have up-to-date values. */
> > - atomic_add_acq_int(&smp_rv_waiters[0], 1);
> > - while (smp_rv_waiters[0] < smp_rv_ncpus)
> > - cpu_spinwait();
> > -
>
> You really need this for architectures that need the memory barrier to
> ensure consistency. We also need to move the reads of smp_rv_* below this
> point to provide a consistent view.
>
> > /* setup function */
> > if (local_setup_func != smp_no_rendevous_barrier) {
> >
> > if (smp_rv_setup_func != NULL)
> >
> > smp_rv_setup_func(smp_rv_func_arg);
> >
> > /* spin on entry rendezvous */
> >
> > - atomic_add_int(&smp_rv_waiters[1], 1);
> > - while (smp_rv_waiters[1] < smp_rv_ncpus)
> > + atomic_add_int(&smp_rv_waiters[0], 1);
> > + while (smp_rv_waiters[0] < smp_rv_ncpus)
> >
> > cpu_spinwait();
> >
> > }
> >
> > @@ -337,12 +332,16 @@
> >
> > local_action_func(local_func_arg);
> >
> > /* spin on exit rendezvous */
> >
> > - atomic_add_int(&smp_rv_waiters[2], 1);
> > - if (local_teardown_func == smp_no_rendevous_barrier)
> > + atomic_add_int(&smp_rv_waiters[1], 1);
> > + if (local_teardown_func == smp_no_rendevous_barrier) {
> > + atomic_add_int(&smp_rv_waiters[2], 1);
> >
> > return;
> >
> > - while (smp_rv_waiters[2] < smp_rv_ncpus)
> > + }
> > + while (smp_rv_waiters[1] < smp_rv_ncpus)
> >
> > cpu_spinwait();
> >
> > + atomic_add_int(&smp_rv_waiters[2], 1);
> > +
> >
> > /* teardown function */
> > if (local_teardown_func != NULL)
> >
> > local_teardown_func(local_func_arg);
> >
> > @@ -377,6 +376,10 @@
> >
> > /* obtain rendezvous lock */
> > mtx_lock_spin(&smp_ipi_mtx);
> >
> > + /* Wait for any previous unwaited rendezvous to finish. */
> > + while (atomic_load_acq_int(&smp_rv_waiters[2]) < smp_rv_ncpus)
> > + cpu_spinwait();
> > +
Disregard this ... I misread the diff. You are indeed using [2] correctly as
the "all-clear" semaphore. I still believe, that it is safer/cleaner to do
this spin before releasing the lock instead (see my patch).
> This does not help you at all. Imagine the following (unlikely, but not
> impossible) case:
>
> CPUA: start rendevouz including self, finish the action first (i.e. CPUA is
> the first one to see smp_rv_waiters[2] == smp_rv_ncpus, drop the lock and
> start a new rendevouz. smp_rv_waiters[2] == smp_rv_ncpus is still true on
> that CPU, but ...
>
> CPUB might have increased smp_rv_waiters[2] for the first rendevouz, but
> never saw smp_rv_waiters[2] == smp_rv_ncpus, still ...
>
> CPUA is allowed to start a new rendevouz which will leave CPUB stranded and
> can lead to a deadlock.
>
> I think this is also possible with another CPU starting the second
> rendevous.
>
> > /* set static function pointers */
> > smp_rv_ncpus = ncpus;
> > smp_rv_setup_func = setup_func;
> >
> > @@ -395,7 +398,7 @@
> >
> > smp_rendezvous_action();
> >
> > if (teardown_func == smp_no_rendevous_barrier)
> >
> > - while (atomic_load_acq_int(&smp_rv_waiters[2]) < ncpus)
> > + while (atomic_load_acq_int(&smp_rv_waiters[1]) < ncpus)
> >
> > cpu_spinwait();
> >
> > /* release lock */
> >
> > _______________________________________________
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