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@xxxxxxx>
---
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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