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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