Re: [Intel-gfx] [PATCH 2/2] dma-buf/dma-fence: Add quick tests before dma_fence_remove_callback

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jul 15, 2020 at 11:49:05AM +0100, Chris Wilson wrote:
> When waiting with a callback on the stack, we must remove the callback
> upon wait completion. Since this will be notified by the fence signal
> callback, the removal often contends with the fence->lock being held by
> the signaler. We can look at the list entry to see if the callback was
> already signaled before we take the contended lock.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/dma-buf/dma-fence.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 8d5bdfce638e..b910d7bc0854 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -420,6 +420,9 @@ dma_fence_remove_callback(struct dma_fence *fence, struct dma_fence_cb *cb)
>  	unsigned long flags;
>  	bool ret;
>  
> +	if (list_empty(&cb->node))

I was about to say "but the races" but then noticed that Paul fixed
list_empty to use READ_ONCE like 5 years ago :-)

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> +		return false;
> +
>  	spin_lock_irqsave(fence->lock, flags);
>  
>  	ret = !list_empty(&cb->node);
> -- 
> 2.20.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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