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

[Samuel Pitoiset]

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).

>
>
>> 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