[Intel-gfx] [PATCH v2] drm/i915: Only prune fences after wait-for-all

[Chris Wilson]

Quoting Matthew Auld (2018-03-08 17:48:48)
> On 7 March 2018 at 17:13, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > Currently, we only allow ourselves to prune the fences so long as
> > all the waits completed (i.e. all the fences we checked were signaled),
> > and that the reservation snapshot did not change across the wait.
> > However, if we only waited for a subset of the reservation object, i.e.
> > just waiting for the last writer to complete as opposed to all readers
> > as well, then we would erroneously conclude we could prune the fences as
> > indeed although all of our waits were successful, they did not represent
> > the totality of the reservation object.
> >
> > v2: We only need to check the shared fences due to construction (i.e.
> > all of the shared fences will be later than the exclusive fence, if
> > any).
> >
> > Fixes: e54ca9774777 ("drm/i915: Remove completed fences after a wait")
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > Cc: Matthew Auld <matthew.auld at intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_gem.c | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index a5bd07338b46..9b48b5101357 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -433,20 +433,28 @@ i915_gem_object_wait_reservation(struct reservation_object *resv,
> >                         dma_fence_put(shared[i]);
> >                 kfree(shared);
> >
> > +               /*
> > +                * If both shared fences and an exclusive fence exist,
> > +                * then by construction the shared fences must be later
> > +                * than the exclusive fence. If we successfully wait for
> > +                * all the shared fences, we know that the exclusive fence
> > +                * must all be signaled. If all the shared fences are
> > +                * signaled, we can prune the array and recover the
> > +                * floating references on the fences/requests.
> > +                */
> >                 prune_fences = count && timeout >= 0;
> >         } else {
> >                 excl = reservation_object_get_excl_rcu(resv);
> >         }
> >
> > -       if (excl && timeout >= 0) {
> > +       if (excl && timeout >= 0)
> >                 timeout = i915_gem_object_wait_fence(excl, flags, timeout,
> >                                                      rps_client);
> > -               prune_fences = timeout >= 0;
> > -       }
> >
> >         dma_fence_put(excl);
> >
> > -       /* Oportunistically prune the fences iff we know they have *all* been
> > +       /*
> > +        * Oportunistically prune the fences iff we know they have *all* been
> Opportunistically
> 
> Reviewed-by: Matthew Auld <matthew.auld at intel.com>

And a bonus spelling fix, thank you!
Pushed,
-Chris