[ovs-dev] [PATCH V3 1/2] ofproto-dpif-upcall: Allow main thread to pause all revalidators.

ALeX Wang ee07b291 at gmail.com
Sun Aug 30 00:42:34 UTC 2015


On 29 August 2015 at 16:45, Joe Stringer <joestringer at nicira.com> wrote:

> On 29 August 2015 at 15:24, Alex Wang <alexw at nicira.com> wrote:
> >
> >
> > On Sat, Aug 29, 2015 at 11:34 AM, Joe Stringer <joestringer at nicira.com>
> > wrote:
> >>
> >> Thanks for working on this, Alex. I've considered implementing an
> >> approach like this before, but haven't had a strong reason to.
> >>
> >> On 29 August 2015 at 00:42, Alex Wang <ee07b291 at gmail.com> wrote:
> >> > This commit adds logic using ovs barrier to allow main thread pause
> >> > all revalidators.  This new feature will be used in a later patch.
> >> >
> >> > Signed-off-by: Alex Wang <ee07b291 at gmail.com>
> >>
> >> <snip>
> >>
> >> > @@ -762,6 +791,11 @@ udpif_revalidator(void *arg)
> >> >
> >> >      revalidator->id = ovsthread_id_self();
> >> >      for (;;) {
> >> > +        /* Pauses all revalidators if wanted. */
> >> > +        if (latch_is_set(&udpif->pause_latch)) {
> >> > +            revalidator_pause(revalidator);
> >> > +        }
> >> > +
> >>
> >> Is there anything that ensures all revalidators are either before this
> >> statement, or after this statement, when the latch is modified?
> >
> >
> > I think this should not matter, since the latch_wait() will cause
> > revalidator
> > waking up immediately...  And latch has only two states (set or not),
> unlike
> > the sequence number, so we do not need to worry about missing the
> > seq_change,
>
> What if the threads don't wake up due to latch_wait(), but due to
> another reason? Each revalidator thread individually checks the value
> at a different time from all the others, so without a critical section
> that ensures they all check the same value, I think it's possible for
> two revalidators to wake up (eg due to timer expiring), one checks the
> latch & skips the barrier, then main thread changes the latch, then
> the second revalidator checks the latch & blocks on the barrier.
>


In that case, I think the first revalidator should proceed to
latch_wait(&udpif->pause_latch);
and wakes up immediately since the latch is set.

At the same time the main thread is still blocking on the barrier,

Thanks,
Alex Wang,

-- 
Alex Wang,
Open vSwitch developer



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