[PATCH] drm/amdgpu: fix old fence check in amdgpu_fence_emit

[Zhou, David(ChunMing)]

> -----Original Message-----
> From: amd-gfx <amd-gfx-bounces at lists.freedesktop.org> On Behalf Of
> Christian K?nig
> Sent: Saturday, March 30, 2019 2:33 AM
> To: amd-gfx at lists.freedesktop.org
> Subject: [PATCH] drm/amdgpu: fix old fence check in amdgpu_fence_emit
> 
> We don't hold a reference to the old fence, so it can go away any time we are
> waiting for it to signal.
> 
> Signed-off-by: Christian König <christian.koenig at amd.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c | 24 ++++++++++++++++-
> ------
>  1 file changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> index ee47c11e92ce..4dee2326b29c 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> @@ -136,8 +136,9 @@ int amdgpu_fence_emit(struct amdgpu_ring *ring,
> struct dma_fence **f,  {
>  	struct amdgpu_device *adev = ring->adev;
>  	struct amdgpu_fence *fence;
> -	struct dma_fence *old, **ptr;
> +	struct dma_fence __rcu **ptr;
>  	uint32_t seq;
> +	int r;
> 
>  	fence = kmem_cache_alloc(amdgpu_fence_slab, GFP_KERNEL);
>  	if (fence == NULL)
> @@ -153,15 +154,24 @@ int amdgpu_fence_emit(struct amdgpu_ring *ring,
> struct dma_fence **f,
>  			       seq, flags | AMDGPU_FENCE_FLAG_INT);
> 
>  	ptr = &ring->fence_drv.fences[seq & ring-
> >fence_drv.num_fences_mask];
> +	if (unlikely(rcu_dereference_protected(*ptr, 1))) {

Isn't this line redundant with dma_fence_get_rcu_safe? I think it's unnecessary.
Otherwise looks ok to me.

-David
> +		struct dma_fence *old;
> +
> +		rcu_read_lock();
> +		old = dma_fence_get_rcu_safe(ptr);
> +		rcu_read_unlock();
> +
> +		if (old) {
> +			r = dma_fence_wait(old, false);
> +			dma_fence_put(old);
> +			if (r)
> +				return r;
> +		}
> +	}
> +
>  	/* This function can't be called concurrently anyway, otherwise
>  	 * emitting the fence would mess up the hardware ring buffer.
>  	 */
> -	old = rcu_dereference_protected(*ptr, 1);
> -	if (old && !dma_fence_is_signaled(old)) {
> -		DRM_INFO("rcu slot is busy\n");
> -		dma_fence_wait(old, false);
> -	}
> -
>  	rcu_assign_pointer(*ptr, dma_fence_get(&fence->base));
> 
>  	*f = &fence->base;
> --
> 2.17.1
> 
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