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

[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);
-- 
2.17.1