[Mesa-dev] [PATCH 1/3] llvmpipe: add lp_fence_timedwait() helper

[Emil Velikov]

On Thu, 11 Apr 2019 at 17:55, Gustaw Smolarczyk <wielkiegie at gmail.com> wrote:
>
> czw., 11 kwi 2019 o 18:06 Emil Velikov <emil.l.velikov at gmail.com> napisał(a):
> >
> > The function is analogous to lp_fence_wait() while taking at timeout
> > (ns) parameter, as needed for EGL fence/sync.
> >
> > Cc: Roland Scheidegger <sroland at vmware.com>
> > Signed-off-by: Emil Velikov <emil.velikov at collabora.com>
> > ---
> >  src/gallium/drivers/llvmpipe/lp_fence.c | 22 ++++++++++++++++++++++
> >  src/gallium/drivers/llvmpipe/lp_fence.h |  3 +++
> >  2 files changed, 25 insertions(+)
> >
> > diff --git a/src/gallium/drivers/llvmpipe/lp_fence.c b/src/gallium/drivers/llvmpipe/lp_fence.c
> > index 20cd91cd63d..f8b31a9d6a5 100644
> > --- a/src/gallium/drivers/llvmpipe/lp_fence.c
> > +++ b/src/gallium/drivers/llvmpipe/lp_fence.c
> > @@ -125,3 +125,25 @@ lp_fence_wait(struct lp_fence *f)
> >  }
> >
> >
> > +boolean
> > +lp_fence_timedwait(struct lp_fence *f, uint64_t timeout)
> > +{
> > +   struct timespec ts = {
> > +      .tv_nsec = timeout % 1000000000L,
> > +      .tv_sec = timeout / 1000000000L,
> > +   };
>
> According to the documentation [1] and looking at the implementation
> in mesa [2], cnd_timedwait accepts an absolute time in UTC, not
> duration. It seems that the fence_finish callback accepts duration.
>
Agreed, not sure how I missed that one.

> [1] https://en.cppreference.com/w/c/thread/cnd_timedwait
> [2] https://gitlab.freedesktop.org/mesa/mesa/blob/master/include/c11/threads_posix.h#L135
>
> > +   int ret;
> > +
> > +   if (LP_DEBUG & DEBUG_FENCE)
> > +      debug_printf("%s %d\n", __FUNCTION__, f->id);
> > +
> > +   mtx_lock(&f->mutex);
> > +   assert(f->issued);
> > +   while (f->count < f->rank) {
> > +      ret = cnd_timedwait(&f->signalled, &f->mutex, &ts);
>
> Shouldn't ret be checked for thrd_busy here as well? Otherwise, the
> function will busy-wait after the timeout is reached instead of
> returning.
>
Actually this should be tweaked to:
 - only wait for the requested timeout
 - _regardless_ of how meany threads (and hence ranks) llvmpipe has

Something like the following (warning pre-coffee code) should work.

   mtx_lock(&f->mutex);
   assert(f->issued);
   if (f->count < f->rank)
      ret = cnd_timedwait(&f->signalled, &f->mutex, &ts);

   mtx_unlock(&f->mutex);
   return (f->count >= f->rank && ret == thrd_success);

Thanks for the review :-)
-Emil