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

[JingWen Chen]

On 2021/10/25 下午11:18, Andrey Grodzovsky wrote:
>
> 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
>
will do
>
>>> 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
>
>
Hi Andrey,

I have tried this and this will cause normal jobs cannot be free(lacks a dma_fence_put). I have figured out all the get/put

for sched_jobs and only the bad job lacks a dma_fence_get, other jobs are just fine.

>>
>>>>> +        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);