linux-kernel - Re: [PATCH] dma-buf: Move BUG_ON from _add_shared_fence to _add_shared

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Date:   Wed, 4 Jul 2018 11:09:58 +0200
From:   Michel Dänzer <michel@...nzer.net>
To:     christian.koenig@....com, Sumit Semwal <sumit.semwal@...aro.org>
Cc:     linaro-mm-sig@...ts.linaro.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        linux-media@...r.kernel.org
Subject: Re: [PATCH] dma-buf: Move BUG_ON from _add_shared_fence to
 _add_shared_inplace

On 2018-07-04 10:31 AM, Christian König wrote:
> Am 26.06.2018 um 16:31 schrieb Michel Dänzer:
>> From: Michel Dänzer <michel.daenzer@....com>
>>
>> Fixes the BUG_ON spuriously triggering under the following
>> circumstances:
>>
>> * ttm_eu_reserve_buffers processes a list containing multiple BOs using
>>    the same reservation object, so it calls
>>    reservation_object_reserve_shared with that reservation object once
>>    for each such BO.
>> * In reservation_object_reserve_shared, old->shared_count ==
>>    old->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
>>    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
>>    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.

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.

-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer