Re: [PATCH] dma-buf: Move BUG_ON from _add_shared_fence to _add_shared_inplace

[Christian KÃnig]

Am 04.07.2018 um 11:09 schrieb Michel DÃnzer:
> On 2018-07-04 10:31 AM, Christian KÃnig wrote:
> > Am 26.06.2018 um 16:31 schrieb Michel DÃnzer:
> > > From: Michel DÃnzer <michel.daenzer@xxxxxxx>
> > >
> > > Fixes the BUG_ON spuriously triggering under the following
> > > circumstances:
> > >
> > > * ttm_eu_reserve_buffers processes a list containing multiple BOs using
> > >  ÂÂ the same reservation object, so it calls
> > >  ÂÂ reservation_object_reserve_shared with that reservation object once
> > >  ÂÂ for each such BO.
> > > * In reservation_object_reserve_shared, old->shared_count ==
> > >  ÂÂ old->shared_max - 1, so obj->staged is freed in preparation of an
> > >  ÂÂ in-place update.
> > > * ttm_eu_fence_buffer_objects calls reservation_object_add_shared_fence
> > >  ÂÂ once for each of the BOs above, always with the same fence.
> > > * The first call adds the fence in the remaining free slot, after which
> > >  ÂÂ old->shared_count == old->shared_max.
> > Well, the explanation here is not correct. For multiple BOs using the
> > same reservation object we won't call
> > reservation_object_add_shared_fence() multiple times because we move
> > those to the duplicates list in ttm_eu_reserve_buffers().
> >
> > But this bug can still happen because we call
> > reservation_object_add_shared_fence() manually with fences for the same
> > context in a couple of places.
> >
> > One prominent case which comes to my mind are for the VM BOs during
> > updates. Another possibility are VRAM BOs which need to be cleared.
> Thanks. How about the following:
>
> * ttm_eu_reserve_buffers calls reservation_object_reserve_shared.
> * In reservation_object_reserve_shared, shared_count == shared_max - 1,
>    so obj->staged is freed in preparation of an in-place update.
> * ttm_eu_fence_buffer_objects calls reservation_object_add_shared_fence,
>    after which shared_count == shared_max.
> * The amdgpu driver also calls reservation_object_add_shared_fence for
>    the same reservation object, and the BUG_ON triggers.

I would rather completely drop the reference to the ttm_eu_* functions, 
cause those wrappers are completely unrelated to the problem.

Instead let's just note something like the following:

* When reservation_object_reserve_shared is called with shared_count == 
shared_max - 1,
 so obj->staged is freed in preparation of an in-place update.

* Now reservation_object_add_shared_fence is called with the first fence 
and after that shared_count == shared_max.

* After that reservation_object_add_shared_fence can be called with 
follow up fences from the same context, but since shared_count == 
shared_max we would run into this BUG_ON.

> However, nothing bad would happen in
> reservation_object_add_shared_inplace, since all fences use the same
> context, so they can only occupy a single slot.
>
> Prevent this by moving the BUG_ON to where an overflow would actually
> happen (e.g. if a buggy caller didn't call
> reservation_object_reserve_shared before).
>
>
> Also, I'll add a reference to https://bugs.freedesktop.org/106418 in v2,
> as I suspect this fix is necessary under the circumstances described
> there as well.

The rest sounds good to me.

Regards,
Christian.