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Subject: Re: [PATCH] dma-buf: Move BUG_ON from _add_shared_fence to
 _add_shared_inplace
To:     =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel@daenzer.net>,
        Sumit Semwal <sumit.semwal@linaro.org>
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        linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org,
        linux-media@vger.kernel.org
References: <20180626143147.14296-1-michel@daenzer.net>
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From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
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Date:   Wed, 4 Jul 2018 11:30:53 +0200
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Am 04.07.2018 um 11:09 schrieb Michel Dänzer:
> 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@amd.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.

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.