[PATCH 1/2] drm/amd/amdgpu: lock reservation object while creating shadow bo

[Christian König]

Am 11.01.2017 um 15:09 schrieb Nicolai Hähnle:
> On 11.01.2017 12:56, Christian König wrote:
>> Am 11.01.2017 um 08:31 schrieb Nicolai Hähnle:
>>> From: Nicolai Hähnle <nicolai.haehnle at amd.com>
>>>
>>> ttm_bo_init checks that the reservation object is locked. This is
>>> the caller's responsibility when resv != NULL. Otherwise, the inline
>>> reservation object of the newly allocated buffer is used and must
>>> explicitly be locked. Using a trylock is fine, since nobody else
>>> has a reference to the reservation lock.
>>
>> Good, catch. But while using trylock is fine, why do you need to use it
>> in the first place?
>
> Or is this a question about trylock vs. lock?
>
> Actually, now that I think about this again, perhaps the following 
> sequence would be possible:
>
> 1. Create the main bo in amdgpu_bo_create.
> 2. Other thread, for whatever reason, tries to make space and evict 
> the bo, and probably locks the reservation for that?
> 3. amdgpu_bo_create continues, calls the trylock which now fails.

Yes, exactly that was my thinking as well.

Please just use the normal uninterruptible locking variant if there 
isn't a good reason for using trylock.

Christian.

>
> Nicolai
>
>>
>> Regards,
>> Christian.
>>
>>>
>>> Signed-off-by: Nicolai Hähnle <nicolai.haehnle at amd.com>
>>> ---
>>>   drivers/gpu/drm/amd/amdgpu/amdgpu_object.c | 11 +++++++++++
>>>   1 file changed, 11 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>> index 11c12c4d..357eed9 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>> @@ -465,21 +465,32 @@ int amdgpu_bo_create(struct amdgpu_device *adev,
>>>       amdgpu_ttm_placement_init(adev, &placement,
>>>                     placements, domain, flags);
>>>         r = amdgpu_bo_create_restricted(adev, size, byte_align, kernel,
>>>                       domain, flags, sg, &placement,
>>>                       resv, bo_ptr);
>>>       if (r)
>>>           return r;
>>>         if (amdgpu_need_backup(adev) && (flags &
>>> AMDGPU_GEM_CREATE_SHADOW)) {
>>> +        if (!resv) {
>>> +            bool locked;
>>> +
>>> +            locked = ww_mutex_trylock(&(*bo_ptr)->tbo.resv->lock);
>>> +            WARN_ON(!locked);
>>> +        }
>>> +
>>>           r = amdgpu_bo_create_shadow(adev, size, byte_align, 
>>> (*bo_ptr));
>>> +
>>> +        if (!resv)
>>> + ww_mutex_unlock(&(*bo_ptr)->tbo.resv->lock);
>>> +
>>>           if (r)
>>>               amdgpu_bo_unref(bo_ptr);
>>>       }
>>>         return r;
>>>   }
>>>     int amdgpu_bo_backup_to_shadow(struct amdgpu_device *adev,
>>>                      struct amdgpu_ring *ring,
>>>                      struct amdgpu_bo *bo,
>>
>>
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