[PATCH 4/4] dma-buf: nuke reservation_object seq number

[Chris Wilson]

Quoting Chris Wilson (2019-08-14 18:38:20)
> Quoting Chris Wilson (2019-08-14 18:22:53)
> > Quoting Chris Wilson (2019-08-14 18:06:18)
> > > Quoting Chris Wilson (2019-08-14 17:42:48)
> > > > Quoting Daniel Vetter (2019-08-14 16:39:08)
> > > > > > > > +       } while (rcu_access_pointer(obj->fence_excl) != *excl);
> > > > > 
> > > > > What if someone is real fast (like really real fast) and recycles the
> > > > > exclusive fence so you read the same pointer twice, but everything else
> > > > > changed? reused fence pointer is a lot more likely than seqlock wrapping
> > > > > around.
> > > > 
> > > > It's an exclusive fence. If it is replaced, it must be later than all
> > > > the shared fences (and dependent on them directly or indirectly), and
> > > > so still a consistent snapshot.
> > > 
> > > An extension of that argument says we don't even need to loop,
> > > 
> > > *list = rcu_dereference(obj->fence);
> > > *shared_count = *list ? (*list)->shared_count : 0;
> > > smp_rmb();
> > > *excl = rcu_dereference(obj->fence_excl);
> > > 
> > > Gives a consistent snapshot. It doesn't matter if the fence_excl is
> > > before or after the shared_list -- if it's after, it's a superset of all
> > > fences, if it's before, we have a correct list of shared fences the
> > > earlier fence_excl.
> > 
> > The problem is that the point of the loop is that we do need a check on
> > the fences after the full memory barrier.
> > 
> > Thinking of the rationale beaten out for dma_fence_get_excl_rcu_safe()
> > 
> > We don't have a full memory barrier here, so this cannot be used safely
> > in light of fence reallocation.
> 
> i.e. we need to restore the loops in the callers,
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_busy.c b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
> index a2aff1d8290e..f019062c8cd7 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_busy.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
> @@ -110,6 +110,7 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
>          * to report the overall busyness. This is what the wait-ioctl does.
>          *
>          */
> +retry:
>         dma_resv_fences(obj->base.resv, &excl, &list, &shared_count);
> 
>         /* Translate the exclusive fence to the READ *and* WRITE engine */
> @@ -122,6 +123,10 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
>                 args->busy |= busy_check_reader(fence);
>         }
> 
> +       smp_rmb();
> +       if (excl != rcu_access_pointer(obj->base.resv->fence_excl))
> +               goto retry;
> +
> 
> wrap that up as
> 
> static inline bool
> dma_resv_fences_retry(struct dma_resv *resv, struct dma_fence *excl)
> {
>         smp_rmb();
>         return excl != rcu_access_pointer(resv->fence_excl);
> }

I give up. It's not just the fence_excl that's an issue here.

Any of the shared fences may be replaced after dma_resv_fences()
and so the original shared fence pointer may be reassigned (even under
RCU). The only defense against that is the seqcount.

I totally screwed that up.
-Chris