Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when slot available

["Christian König" <christian.koenig@amd.com>]

Am 05.03.2018 um 12:37 schrieb Liu, Monk:
> But the thing confuse me is according to the design, if driver keep calling reserve_shared() prior to add_fence(), and with lock held of cause,
> That BUG() shouldn't hit, so there are two things in face looks weired to me:
> 1) by design in reserve_shared(), obj->staged should be already NULL, so why we kfree on it

No, that is not correct.

> 2) in fact, amdgpu can hit the case that obj->staged is not NULL in reserved_shared(), don't know how it lead here

We reserved a fence slot without using it, so it is still there when 
reserve_shared() is called.

Christian.

>
>
> Any thought ?
>
> /Monk
>
> -----Original Message-----
> From: Koenig, Christian
> Sent: 2018年3月5日 19:29
> To: Liu, Monk <Monk.Liu@amd.com>; dri-devel@lists.freedesktop.org; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when slot available
>
> Am 05.03.2018 um 12:25 schrieb Liu, Monk:
>> And by the way, I add "if (staged!=NULL) BUG();" prior to
>> "kfree(obj->staged)" in reserve_shared() routine, and this BUG() is actually hit, The stack dump shows it is hit during the vm_bo_update() in gem_va_update()...
> That is expected. The staged handling just makes sure that there is room available, it doesn't guarantee that it is actually used.
>
> E.g. we can end up reserving a fence slot, but then find that we actually don't need it.
>
> Christian.
>
>> Besides, the whole reservation logic still looks a little weired to me ... especially this staged part ...
>>
>> Thanks
>>
>> /Monk
>>
>> -----Original Message-----
>> From: Christian König [mailto:ckoenig.leichtzumerken@gmail.com]
>> Sent: 2018年3月5日 19:22
>> To: Liu, Monk <Monk.Liu@amd.com>; Koenig, Christian
>> <Christian.Koenig@amd.com>; dri-devel@lists.freedesktop.org;
>> linux-kernel@vger.kernel.org
>> Subject: Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when
>> slot available
>>
>> Am 05.03.2018 um 08:55 schrieb Liu, Monk:
>>> Hi Christian
>>>
>>> You are right on that part of obj-staged is set to NULL in add_fence,
>>> So my following question will be why we kfree(obj->staged) in reserve_shared() if staged is always NULL in that point ?
>> Good question, I haven't wrote code that so I can't fully answer.
>>
>> Maybe Chris or Maarten know more about that.
>>
>> Christian.
>>
>>> Thanks
>>> /Monk
>>>
>>> -----Original Message-----
>>> From: Christian König [mailto:ckoenig.leichtzumerken@gmail.com]
>>> Sent: 2018年2月28日 16:27
>>> To: Liu, Monk <Monk.Liu@amd.com>; dri-devel@lists.freedesktop.org;
>>> linux-kernel@vger.kernel.org
>>> Subject: Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when
>>> slot available
>>>
>>> Am 28.02.2018 um 07:44 schrieb Monk Liu:
>>>> under below scenario the obj->fence would refer to a wild pointer:
>>>>
>>>> 1,call reservation_object_reserved_shared
>>>> 2,call reservation_object_add_shared_fence
>>>> 3,call reservation_object_reserved_shared
>>>> 4,call reservation_object_add_shared_fence
>>>>
>>>> in step 1, staged is allocated,
>>>>
>>>> in step 2, code path will go reservation_object_add_shared_replace()
>>>> and obj->fence would be assigned as staged (through
>>>> RCU_INIT_POINTER)
>>>>
>>>> in step 3, obj->staged will be freed(by simple kfree), which make
>>>> obj->fence point to a wild pointer...
>>> Well that explanation is still nonsense. See
>>> reservation_object_add_shared_fence:
>>>>            obj->staged = NULL;
>>> Among the first things reservation_object_add_shared_fence() does is
>>> it sets obj->staged to NULL.
>>>
>>> So step 3 will not free anything and we never have a wild pointer.
>>>
>>> Regards,
>>> Christian.
>>>
>>>> in step 4, code path will go reservation_object_add_shared_inplace()
>>>> and inside it the @fobj (which equals to @obj->staged, set by above
>>>> steps) is already a wild pointer
>>>>
>>>> should remov the kfree on staged in
>>>> reservation_object_reserve_shared()
>>>>
>>>> Change-Id: If7c01f1b4be3d3d8a81efa90216841f79ab1fc1c
>>>> Signed-off-by: Monk Liu <Monk.Liu@amd.com>
>>>> ---
>>>>      drivers/dma-buf/reservation.c | 7 ++-----
>>>>      1 file changed, 2 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/dma-buf/reservation.c
>>>> b/drivers/dma-buf/reservation.c index 375de41..b473ccc 100644
>>>> --- a/drivers/dma-buf/reservation.c
>>>> +++ b/drivers/dma-buf/reservation.c
>>>> @@ -74,12 +74,9 @@ int reservation_object_reserve_shared(struct reservation_object *obj)
>>>>      	old = reservation_object_get_list(obj);
>>>>      
>>>>      	if (old && old->shared_max) {
>>>> -		if (old->shared_count < old->shared_max) {
>>>> -			/* perform an in-place update */
>>>> -			kfree(obj->staged);
>>>> -			obj->staged = NULL;
>>>> +		if (old->shared_count < old->shared_max)
>>>>      			return 0;
>>>> -		} else
>>>> +		else
>>>>      			max = old->shared_max * 2;
>>>>      	} else
>>>>      		max = 4;
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