[PATCH v2 1/2] drm/amdgpu: fix a potential deadlock in amdgpu_bo_create_restricted()

[Samuel Pitoiset]

On 02/13/2017 07:19 PM, Nicolai Hähnle wrote:
> On 13.02.2017 19:11, Samuel Pitoiset wrote:
>>
>>
>> On 02/13/2017 07:04 PM, Nicolai Hähnle wrote:
>>> On 13.02.2017 18:49, Samuel Pitoiset wrote:
>>>>
>>>>
>>>> On 02/13/2017 05:25 PM, Nicolai Hähnle wrote:
>>>>> On 09.02.2017 11:33, Samuel Pitoiset wrote:
>>>>>> When ttm_bo_init() fails, the reservation mutex should be unlocked.
>>>>>>
>>>>>> In debug build, the kernel reported "possible recursive locking
>>>>>> detected" in this codepath. For debugging purposes, I also added
>>>>>> a "WARN_ON(ww_mutex_is_locked())" when ttm_bo_init() fails and the
>>>>>> mutex was locked as expected.
>>>>>>
>>>>>> This should fix (random) GPU hangs. The easy way to reproduce the
>>>>>> issue is to change the "Super Sampling" option from 1.0 to 2.0 in
>>>>>> Hitman. It will create a huge buffer, evict a bunch of buffers
>>>>>> (around ~5k) and deadlock.
>>>>>>
>>>>>> This regression has been introduced pretty recently.
>>>>>>
>>>>>> v2: only release the mutex if resv is NULL
>>>>>>
>>>>>> Fixes: 12a852219583 ("drm/amdgpu: improve
>>>>>> AMDGPU_GEM_CREATE_VRAM_CLEARED handling (v2)")
>>>>>> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
>>>>>> ---
>>>>>>  drivers/gpu/drm/amd/amdgpu/amdgpu_object.c | 5 ++++-
>>>>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>>>>> index d1ef1d064de4..556236a112c1 100644
>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>>>>> @@ -403,8 +403,11 @@ int amdgpu_bo_create_restricted(struct
>>>>>> amdgpu_device *adev,
>>>>>>              &bo->placement, page_align, !kernel, NULL,
>>>>>>              acc_size, sg, resv ? resv : &bo->tbo.ttm_resv,
>>>>>>              &amdgpu_ttm_bo_destroy);
>>>>>> -    if (unlikely(r != 0))
>>>>>> +    if (unlikely(r != 0)) {
>>>>>> +        if (!resv)
>>>>>> +            ww_mutex_unlock(&bo->tbo.resv->lock);
>>>>>>          return r;
>>>>>> +    }
>>>>>
>>>>> I was looking at this myself a couple of weeks back, and I'm pretty
>>>>> sure
>>>>> I had this exact same patch just to realize that it's actually
>>>>> incorrect.
>>>>>
>>>>> The problem is that ttm_bo_init will actually call the destroy
>>>>> function
>>>>> (in our case, amdgpu_ttm_bo_destroy), so at this point, bo has been
>>>>> freed.
>>>>>
>>>>> This code is a huge mess. I'm surprised though: have you verified that
>>>>> this patch actually fixes a hang?
>>>>
>>>> Yes, I triple-checked. I can't reproduce the hangs with Hitman.
>>>
>>> That's surprising, but a relief. Maybe it ties into some of the other
>>> problems I'm seeing as well.
>>>
>>> This means we need a real fix for this; I still think the current patch
>>> is broken.
>>
>> Maybe the issue is somewhere else and this not the proper solution, but
>> I don't think the given patch is broken as-is. It fixes deadlocks which
>> are pretty easy to reproduce with Hitman (as explained in the commit
>> description).
>
> I'm sorry, but a use-after-free is clearly broken.

You are right. If the destroy callback is called, there is a 
use-after-free which is bad, really..

>
> Nicolai
>
>>
>>>
>>>
>>>> This fixes a deadlock, here's the report:
>>>> https://hastebin.com/durodivoma.xml
>>>>
>>>> The resv->lock has to be unlocked when ttm_bo_init() fails (I checked
>>>> with a WARN_ON(is_locked)) because it doesn't call the destroy function
>>>> in all situations. Presumably, when drm_vma_offset_add() fails and resv
>>>> is not NULL, the mutex is not unlocked.
>>>
>>> On which code path is the destroy function not called? If that is the
>>> case, we're leaking memory.
>>>
>>> With the patch as-is, the error paths are either leaking memory (if
>>> you're right) or accessing memory after it's freed (otherwise).
>>> Obviously, neither is good.
>>
>> No, I was wrong. resv is always NULL in this situation. The best
>> solution is probably to try to clean up that code path because I do
>> agree: it's a bit messy.
>>
>>>
>>> Nicolai
>