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

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

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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> on behalf of 
> Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
> *Sent:* Tuesday, March 24, 2020 11:40:06 AM
> *To:* Tao, Yintian <Yintian.Tao@amd.com>; Koenig, Christian 
> <Christian.Koenig@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
>
> 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>
> > ---
> >   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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