Re: [PATCH] drm/amdgpu: hold the reference of finished fence

["Grodzovsky, Andrey" <Andrey.Grodzovsky@amd.com>]

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I see now. In that case the change seems good to me.

Andrey
________________________________
From: Koenig, Christian <Christian.Koenig@amd.com>
Sent: 24 March 2020 13:58
To: Grodzovsky, Andrey <Andrey.Grodzovsky@amd.com>; Pan, Xinhui <Xinhui.Pan@amd.com>; Tao, Yintian <Yintian.Tao@amd.com>; Deucher, Alexander <Alexander.Deucher@amd.com>
Cc: amd-gfx@lists.freedesktop.org <amd-gfx@lists.freedesktop.org>
Subject: Re: [PATCH] drm/amdgpu: hold the reference of finished fence

There is a misunderstanding here:

Did you find out why the zero refcount on the finished fence happens
before the fence was signaled ?

The refcount on the finished fence doesn't become zero before it is signaled, it becomes zero while it is signaled.

CPU 1 calls dma_fence_signal(fence) without holding a reference to the fence. CPU 2 at the same time checks if the fence is signaled and frees the last reference because it find the signaled flag to be set.

The problem is now that dma_fence_signal() wants to set the timestamp after setting the signaled flag and now races with freeing the memory.

That's a really hard to hit problem, but it indeed seems to be possible.

Christian.

Am 24.03.20 um 15:52 schrieb Andrey Grodzovsky:

This is only for the guilty job which was removed from the ring_mirror_list due to completion and hence will not be resubmitted by recovery and will not be freed by the usual flow in drm_sched_get_cleanup_job (see drm_sched_stop)

Andrey

On 3/24/20 10:45 AM, Pan, Xinhui wrote:

[AMD Official Use Only - Internal Distribution Only]

Does this issue occur when gpu recovery?
I just check the code,  fence timedout will free job and put its fence. but gpu recovery might resubmit job.
Correct me if I am wrong.
________________________________
From: amd-gfx <amd-gfx-bounces@lists.freedesktop.org><mailto:amd-gfx-bounces@lists.freedesktop.org> on behalf of Andrey Grodzovsky <Andrey.Grodzovsky@amd.com><mailto:Andrey.Grodzovsky@amd.com>
Sent: Tuesday, March 24, 2020 11:40:06 AM
To: Tao, Yintian <Yintian.Tao@amd.com><mailto:Yintian.Tao@amd.com>; Koenig, Christian <Christian.Koenig@amd.com><mailto:Christian.Koenig@amd.com>; Deucher, Alexander <Alexander.Deucher@amd.com><mailto:Alexander.Deucher@amd.com>
Cc: amd-gfx@lists.freedesktop.org<mailto:amd-gfx@lists.freedesktop.org> <amd-gfx@lists.freedesktop.org><mailto:amd-gfx@lists.freedesktop.org>
Subject: Re: [PATCH] drm/amdgpu: hold the reference of finished fence


On 3/23/20 10:22 AM, Yintian Tao wrote:
> There is one one corner case at dma_fence_signal_locked
> which will raise the NULL pointer problem just like below.
> ->dma_fence_signal
>      ->dma_fence_signal_locked
>        ->test_and_set_bit
> here trigger dma_fence_release happen due to the zero of fence refcount.


Did you find out why the zero refcount on the finished fence happens
before the fence was signaled ? The finished fence is created with
refcount set to 1 in drm_sched_fence_create->dma_fence_init and then the
refcount is decremented in
drm_sched_main->amdgpu_job_free_cb->drm_sched_job_cleanup. This should
only happen after fence is already signaled (see
drm_sched_get_cleanup_job). On top of that the finished fence is
referenced from other places (e.g. entity->last_scheduled e.t.c)...


>
> ->dma_fence_put
>      ->dma_fence_release
>        ->drm_sched_fence_release_scheduled
>            ->call_rcu
> here make the union fled “cb_list” at finished fence
> to NULL because struct rcu_head contains two pointer
> which is same as struct list_head cb_list
>
> Therefore, to hold the reference of finished fence at drm_sched_process_job
> to prevent the null pointer during finished fence dma_fence_signal
>
> [  732.912867] BUG: kernel NULL pointer dereference, address: 0000000000000008
> [  732.914815] #PF: supervisor write access in kernel mode
> [  732.915731] #PF: error_code(0x0002) - not-present page
> [  732.916621] PGD 0 P4D 0
> [  732.917072] Oops: 0002 [#1] SMP PTI
> [  732.917682] CPU: 7 PID: 0 Comm: swapper/7 Tainted: G           OE     5.4.0-rc7 #1
> [  732.918980] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
> [  732.920906] RIP: 0010:dma_fence_signal_locked+0x3e/0x100
> [  732.938569] Call Trace:
> [  732.939003]  <IRQ>
> [  732.939364]  dma_fence_signal+0x29/0x50
> [  732.940036]  drm_sched_fence_finished+0x12/0x20 [gpu_sched]
> [  732.940996]  drm_sched_process_job+0x34/0xa0 [gpu_sched]
> [  732.941910]  dma_fence_signal_locked+0x85/0x100
> [  732.942692]  dma_fence_signal+0x29/0x50
> [  732.943457]  amdgpu_fence_process+0x99/0x120 [amdgpu]
> [  732.944393]  sdma_v4_0_process_trap_irq+0x81/0xa0 [amdgpu]
>
> v2: hold the finished fence at drm_sched_process_job instead of
>      amdgpu_fence_process
> v3: resume the blank line
>
> Signed-off-by: Yintian Tao <yttao@amd.com><mailto:yttao@amd.com>
> ---
>   drivers/gpu/drm/scheduler/sched_main.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index a18eabf692e4..8e731ed0d9d9 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -651,7 +651,9 @@ static void drm_sched_process_job(struct dma_fence *f, struct dma_fence_cb *cb)
>
>        trace_drm_sched_process_job(s_fence);
>
> +     dma_fence_get(&s_fence->finished);
>        drm_sched_fence_finished(s_fence);


If the fence was already released during call to
drm_sched_fence_finished->dma_fence_signal->... why is it safe to
reference the s_fence just before that call ? Can't it already be
released by this time ?

Andrey



> +     dma_fence_put(&s_fence->finished);
>        wake_up_interruptible(&sched->wake_up_worker);
>   }
>
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