Re: [PATCH 2/2] dma-buf/dma-fence: Order setting of the signaled-bit properly

[Christian König]

On 10/21/26 11:55, Philipp Stanner wrote:
> dma_fence_is_signaled() can return true *while* the fence is still
> signaling. This is fragile and has been involved in at least two bugs:
> 
> 	commit c8a5d5ea3ba6 ("nouveau: fix client work fence deletion race")
> 	commit 7c703a7d3f2b ("drm/amdgpu: Fix one use-after-free of VM")
> 
> The signaled-bit is set under lock-protection but read without it.
> Adding the lock has been rejected on the grounds of the lock-acquisition
> shooting down cache lines of parties that merely want to check whether
> the fence is already signaled.
> 
> Make sure the bit can only become visible as 1 once the callbacks have
> ran. To achieve this, read the bit with test_bit_acquire() and set it
> with set_bit_release().

That has been suggested and rejected before.

Background is that the signaling bit must be visible before the callbacks are run or things like dma_fence_default_wait() (as an example) just break.

Regards,
Christian.

> 
> Signed-off-by: Philipp Stanner <phasta@xxxxxxxxxx>
> ---
>  drivers/dma-buf/dma-fence.c | 11 +++++++++--
>  include/linux/dma-fence.h   |  6 +++++-
>  2 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index bd58688b81a7..07d499fa35ac 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -359,8 +359,7 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence,
>  
>  	dma_fence_assert_held(fence);
>  
> -	if (unlikely(test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> -				      &fence->flags)))
> +	if (unlikely(dma_fence_test_signaled_flag(fence)))
>  		return;
>  
>  	trace_dma_fence_signaled(fence);
> @@ -384,6 +383,14 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence,
>  		INIT_LIST_HEAD(&cur->node);
>  		cur->func(fence, cur);
>  	}
> +
> +	/*
> +	 * This orders the signaled-state with the test_bit_acquire() in
> +	 * dma_fence_test_signaled_flag(). This ensures that functions like
> +	 * dma_fence_is_signaled() can only ever return true once signaling is
> +	 * completed (i.e., after the callbacks above ran).
> +	 */
> +	set_bit_release(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags);
>  }
>  EXPORT_SYMBOL(dma_fence_signal_timestamp_locked);
>  
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index dd07d128adc3..61522027af41 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -496,7 +496,11 @@ const char __rcu *dma_fence_timeline_name(struct dma_fence *fence);
>  static inline bool
>  dma_fence_test_signaled_flag(struct dma_fence *fence)
>  {
> -	return test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags);
> +	/*
> +	 * The counterpart of this ordered operation can be found in
> +	 * dma_fence_signal_timestamp_locked().
> +	 */
> +	return test_bit_acquire(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags);
>  }
>  
>  /**