Re: [PATCH 2/2] opensm: Protect against spurious wakeups when calling cl_event_wait_on

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Thu, Nov 01, 2012 at 12:59:58AM -0700, Roland Dreier wrote:
> On Mon, Oct 29, 2012 at 4:45 PM, Albert Chu <chu11-i2BcT+NCU+M@public.gmane.org> wrote:
> > @@ -525,8 +525,8 @@ static void cc_poller_send(osm_congestion_control_t *p_cc,
> >         status = osm_vendor_send(p_cc->bind_handle, p_madw, TRUE);
> >         if (status == IB_SUCCESS) {
> >                 cl_atomic_inc(&p_cc->outstanding_mads_on_wire);
> > -               if (p_cc->outstanding_mads_on_wire >
> > -                   (int32_t)p_opt->cc_max_outstanding_mads)
> > +               while (p_cc->outstanding_mads_on_wire >
> > +                      (int32_t)p_opt->cc_max_outstanding_mads)
> >                         cl_event_wait_on(&p_cc->sig_mads_on_wire_continue,
> >                                          EVENT_NO_TIMEOUT,
> >                                          TRUE);
> 
> I've never looked at the opensm code -- I'm just guessing based on this patch.

The event objects have a hidden built in state that ensures a wake up
is not lost, so long as only one thread ever calls wait_on. If it is
possible two threads could be sleeping on the same event then the
system is unfixably-broken-by-design, since on thread will eat the
internal event and the other will thus miss it, in a racy way.

I've had to clean this kind of a mess up in other code bases, and now
always discourage this kind of interface. Use POSIX condition
variables, they have cleaner locking semantics and are easier to audit
for correctness.

Jason
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