Re: [PATCH] drm/i915: Move fd_install after last use of fence

[Tvrtko Ursulin]

On 03/02/2023 18:15, Rob Clark wrote:
> On Fri, Feb 3, 2023 at 8:49 AM Rob Clark <robdclark@xxxxxxxxx> wrote:
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > Because eb_composite_fence_create() drops the fence_array reference
> > after creation of the sync_file, only the sync_file holds a ref to the
> > fence.  But fd_install() makes that reference visable to userspace, so
> > it must be the last thing we do with the fence.
>
> Fixes: 00dae4d3d35d ("drm/i915: Implement SINGLE_TIMELINE with a syncobj (v4)")

This is correct and the fix looks good to me.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

CI is green so I will merge it, thanks again for a fix Rob!

Followup up question for Matthew Brost however is whether the composite 
fence flow could be simplified. This block here comes late in 
i915_gem_do_execbuffer and may mislead the user the composite fence is 
held to the end of the function:

	if (!out_fence && eb.composite_fence)
		dma_fence_put(eb.composite_fence);

Question is would it work to remove the !out_fence condition from here, 
and remove "consumption" of the reference from eb_composite_fence_create 
success path.

Regards,

Tvrtko

> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 14 +++++++-------
> >   1 file changed, 7 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > index f266b68cf012..0f2e056c02dd 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > @@ -3476,38 +3476,38 @@ i915_gem_do_execbuffer(struct drm_device *dev,
> >
> >   err_request:
> >          eb_requests_get(&eb);
> >          err = eb_requests_add(&eb, err);
> >
> >          if (eb.fences)
> >                  signal_fence_array(&eb, eb.composite_fence ?
> >                                     eb.composite_fence :
> >                                     &eb.requests[0]->fence);
> >
> > +       if (unlikely(eb.gem_context->syncobj)) {
> > +               drm_syncobj_replace_fence(eb.gem_context->syncobj,
> > +                                         eb.composite_fence ?
> > +                                         eb.composite_fence :
> > +                                         &eb.requests[0]->fence);
> > +       }
> > +
> >          if (out_fence) {
> >                  if (err == 0) {
> >                          fd_install(out_fence_fd, out_fence->file);
> >                          args->rsvd2 &= GENMASK_ULL(31, 0); /* keep in-fence */
> >                          args->rsvd2 |= (u64)out_fence_fd << 32;
> >                          out_fence_fd = -1;
> >                  } else {
> >                          fput(out_fence->file);
> >                  }
> >          }
> >
> > -       if (unlikely(eb.gem_context->syncobj)) {
> > -               drm_syncobj_replace_fence(eb.gem_context->syncobj,
> > -                                         eb.composite_fence ?
> > -                                         eb.composite_fence :
> > -                                         &eb.requests[0]->fence);
> > -       }
> > -
> >          if (!out_fence && eb.composite_fence)
> >                  dma_fence_put(eb.composite_fence);
> >
> >          eb_requests_put(&eb);
> >
> >   err_vma:
> >          eb_release_vmas(&eb, true);
> >          WARN_ON(err == -EDEADLK);
> >          i915_gem_ww_ctx_fini(&eb.ww);
> >
> > --
> > 2.38.1