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

[Christian König]

I've just double checked and your analyses actually can't be correct.

When we call dma_fence_signal() in amdgpu_fence_process() we still have 
a reference to the fence.

See the code here:
>                 r = dma_fence_signal(fence);
>                 if (!r)
>                         DMA_FENCE_TRACE(fence, "signaled from irq 
> context\n");
>                 else
>                         BUG();
>
>                 dma_fence_put(fence);

So I'm not sure how you ran into the crash in the first place, this is 
most likely something else.

Regards,
Christian.

Am 23.03.20 um 12:49 schrieb Yintian Tao:
> 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.
>
> ->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 amdgpu_job_run
> to prevent the null pointer during 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]
>
> Signed-off-by: Yintian Tao <yttao at amd.com>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c | 19 ++++++++++++++++++-
>   drivers/gpu/drm/amd/amdgpu/amdgpu_job.c   |  2 ++
>   drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h  |  3 +++
>   3 files changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> index 7531527067df..03573eff660a 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> @@ -52,7 +52,7 @@
>   
>   struct amdgpu_fence {
>   	struct dma_fence base;
> -
> +	struct dma_fence *finished;
>   	/* RB, DMA, etc. */
>   	struct amdgpu_ring		*ring;
>   };
> @@ -149,6 +149,7 @@ int amdgpu_fence_emit(struct amdgpu_ring *ring, struct dma_fence **f,
>   
>   	seq = ++ring->fence_drv.sync_seq;
>   	fence->ring = ring;
> +	fence->finished = NULL;
>   	dma_fence_init(&fence->base, &amdgpu_fence_ops,
>   		       &ring->fence_drv.lock,
>   		       adev->fence_context + ring->idx,
> @@ -182,6 +183,21 @@ int amdgpu_fence_emit(struct amdgpu_ring *ring, struct dma_fence **f,
>   	return 0;
>   }
>   
> +void amdgpu_fence_get_finished(struct dma_fence *base,
> +			       struct dma_fence *finished)
> +{
> +	struct amdgpu_fence *afence = to_amdgpu_fence(base);
> +
> +	afence->finished = dma_fence_get(finished);
> +}
> +
> +void amdgpu_fence_put_finished(struct dma_fence *base)
> +{
> +	struct amdgpu_fence *afence = to_amdgpu_fence(base);
> +
> +	dma_fence_put(afence->finished);
> +}
> +
>   /**
>    * amdgpu_fence_emit_polling - emit a fence on the requeste ring
>    *
> @@ -276,6 +292,7 @@ bool amdgpu_fence_process(struct amdgpu_ring *ring)
>   			BUG();
>   
>   		dma_fence_put(fence);
> +		amdgpu_fence_put_finished(fence);
>   		pm_runtime_mark_last_busy(adev->ddev->dev);
>   		pm_runtime_put_autosuspend(adev->ddev->dev);
>   	} while (last_seq != seq);
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c
> index 4981e443a884..deb2aeeadfb3 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c
> @@ -229,6 +229,8 @@ static struct dma_fence *amdgpu_job_run(struct drm_sched_job *sched_job)
>   				       &fence);
>   		if (r)
>   			DRM_ERROR("Error scheduling IBs (%d)\n", r);
> +		else
> +			amdgpu_fence_get_finished(fence, finished);
>   	}
>   	/* if gpu reset, hw fence will be replaced here */
>   	dma_fence_put(job->fence);
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h
> index 448c76cbf3ed..fd4da91859aa 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h
> @@ -96,6 +96,9 @@ void amdgpu_fence_driver_suspend(struct amdgpu_device *adev);
>   void amdgpu_fence_driver_resume(struct amdgpu_device *adev);
>   int amdgpu_fence_emit(struct amdgpu_ring *ring, struct dma_fence **fence,
>   		      unsigned flags);
> +void amdgpu_fence_get_finished(struct dma_fence *base,
> +			       struct dma_fence *finished);
> +void amdgpu_fence_put_finished(struct dma_fence *base);
>   int amdgpu_fence_emit_polling(struct amdgpu_ring *ring, uint32_t *s);
>   bool amdgpu_fence_process(struct amdgpu_ring *ring);
>   int amdgpu_fence_wait_empty(struct amdgpu_ring *ring);