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

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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@xxxxxxx>
---
  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);

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