[PATCH] drm/amd/amdgpu: fix potential bad job hw_fence underflow

[Andrey Grodzovsky]

On 2021-10-24 10:56 p.m., JingWen Chen wrote:
> On 2021/10/23 上午4:41, Andrey Grodzovsky wrote:
>> What do you mean by underflow in this case ? You mean use after free because of extra dma_fence_put() ?
> yes


Then maybe update the description  because 'underflow' is very confusing


>> On 2021-10-22 4:14 a.m., JingWen Chen wrote:
>>> ping
>>>
>>> On 2021/10/22 AM11:33, Jingwen Chen wrote:
>>>> [Why]
>>>> In advance tdr mode, the real bad job will be resubmitted twice, while
>>>> in drm_sched_resubmit_jobs_ext, there's a dma_fence_put, so the bad job
>>>> is put one more time than other jobs.
>>>>
>>>> [How]
>>>> Adding dma_fence_get before resbumit job in
>>>> amdgpu_device_recheck_guilty_jobs and put the fence for normal jobs
>>>>
>>>> Signed-off-by: Jingwen Chen <Jingwen.Chen2 at amd.com>
>>>> ---
>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_device.c | 4 ++++
>>>>    1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
>>>> index 41ce86244144..975f069f6fe8 100644
>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
>>>> @@ -4841,6 +4841,9 @@ static void amdgpu_device_recheck_guilty_jobs(
>>>>              /* clear job's guilty and depend the folowing step to decide the real one */
>>>>            drm_sched_reset_karma(s_job);
>>>> +        /* for the real bad job, it will be resubmitted twice, adding a dma_fence_get
>>>> +         * to make sure fence is balanced */
>>
>> But that put in drm_sched_resubmit_jobs_ext is for the previous parent fence.
>> fence = sched->ops->run_job(s_job); returns a new HW fence and the put drops the refcount on the old one.
>>
>> Andrey
>>
>>
> Hi Andrey,
>
> If I remember correctly, after we embedded the hw_fence into amdgpu_job, there will be not fence replacement in amdgpu_job_run.


Right, I forgot that... What about removing line 
https://elixir.bootlin.com/linux/v5.15-rc6/source/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c#L265 
?
What if you make dma_get_fence unconditional instead ?

Andrey


>
>>>> +        dma_fence_get(s_job->s_fence->parent);
>>>>            drm_sched_resubmit_jobs_ext(&ring->sched, 1);
>>>>              ret = dma_fence_wait_timeout(s_job->s_fence->parent, false, ring->sched.timeout);
>>>> @@ -4876,6 +4879,7 @@ static void amdgpu_device_recheck_guilty_jobs(
>>>>              /* got the hw fence, signal finished fence */
>>>>            atomic_dec(ring->sched.score);
>>>> +        dma_fence_put(s_job->s_fence->parent);
>>>>            dma_fence_get(&s_job->s_fence->finished);
>>>>            dma_fence_signal(&s_job->s_fence->finished);
>>>>            dma_fence_put(&s_job->s_fence->finished);