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

[Nicolai Hähnle]

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.


> 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.

Nicolai