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

[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 at gmail.com>
发送时间: 2021年9月11日 15:45
收件人: Pan, Xinhui; amd-gfx at 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 at 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);