Re: 回复: [PATCH] drm/amdgpu: Fix a race of IB test

["Christian König" <christian.koenig@amd.com>]

Well is the delayed init work using direct submission or submission 
through the scheduler?

If the later we have the down_write of the reset semaphore pulled in 
through the scheduler dependency.

Anyway just having the sync before taking the lock should work.

Christian.

Am 11.09.21 um 12:18 schrieb Pan, Xinhui:
> [AMD Official Use Only]
>
> For the possible deadlock, we can just move flush_delayed_work above down_write. not a big thing.
> But I am not aware why delayed init work would try to lock reset_sem.
>
> delayed init work is enqueued when device resume. It calls amdgpu_ib_ring_tests directly. We need one sync method.
> But I see device resume itself woud flush it. So there is no race between them as userspace is still freezed.
>
> I will drop this flush in V2.
> ________________________________________
> 发件人: Christian König <ckoenig.leichtzumerken@gmail.com>
> 发送时间: 2021年9月11日 15:45
> 收件人: Pan, Xinhui; amd-gfx@lists.freedesktop.org
> 抄送: Deucher, Alexander; Koenig, Christian
> 主题: Re: [PATCH] drm/amdgpu: Fix a race of IB test
>
>
>
> Am 11.09.21 um 03:55 schrieb xinhui pan:
>> Direct IB submission should be exclusive. So use write lock.
>>
>> Signed-off-by: xinhui pan <xinhui.pan@amd.com>
>> ---
>>    drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c | 9 +++++++--
>>    1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
>> index 19323b4cce7b..acbe02928791 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
>> @@ -1358,10 +1358,15 @@ static int amdgpu_debugfs_test_ib_show(struct seq_file *m, void *unused)
>>        }
>>
>>        /* Avoid accidently unparking the sched thread during GPU reset */
>> -     r = down_read_killable(&adev->reset_sem);
>> +     r = down_write_killable(&adev->reset_sem);
>>        if (r)
>>                return r;
>>
>> +     /* Avoid concurrently IB test but not cancel it as I don't know whether we
>> +      * would add more code in the delayed init work.
>> +      */
>> +     flush_delayed_work(&adev->delayed_init_work);
>> +
> That won't work. It's at least theoretical possible that the delayed
> init work waits for the reset_sem which we are holding here.
>
> Very unlikely to happen, but lockdep might be able to point that out
> with a nice backtrace in the logs.
>
> On the other hand delayed init work and direct IB test through this
> interface should work at the same time, so I would just drop it.
>
> Christian.
>
>>        /* hold on the scheduler */
>>        for (i = 0; i < AMDGPU_MAX_RINGS; i++) {
>>                struct amdgpu_ring *ring = adev->rings[i];
>> @@ -1387,7 +1392,7 @@ static int amdgpu_debugfs_test_ib_show(struct seq_file *m, void *unused)
>>                kthread_unpark(ring->sched.thread);
>>        }
>>
>> -     up_read(&adev->reset_sem);
>> +     up_write(&adev->reset_sem);
>>
>>        pm_runtime_mark_last_busy(dev->dev);
>>        pm_runtime_put_autosuspend(dev->dev);