Re: [RFC PATCH] dma-buf/dma_fence: Make races for dma_fence_is_signaled() less likely

["Gary Guo" <gary@garyguo.net>]

On Fri Jun 12, 2026 at 11:42 AM BST, Philipp Stanner wrote:
> dma_fence_is_signaled() returns whether a fence has been signaled
> already. That function contains a fast path opportunistic check which is
> not guarded by the lock and, according to Christian, cannot be guarded
> by the lock without causing a massive performance regression.
>
> This now means that dma_fence_is_signaled() can return true WHILE the
> fence callbacks are still being executed. This is razy and has lead to
> at least one bug solved in:
>
> commit c8a5d5ea3ba6 ("nouveau: fix client work fence deletion race")
>
> Make this race impossible, by simply setting the bit only once the
> callbacks are actually completed.
>
> Signed-off-by: Philipp Stanner <phasta@kernel.org>
> ---
>  drivers/dma-buf/dma-fence.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index c7ea1e75d38a..2416cc86ce93 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -359,8 +359,19 @@ 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)))
> +	/*
> +	 * First test the bit, so we don't signal an already signaled fence again.
> +	 * The lock protects against multiple parties setting the bit. The bit
> +	 * is then set at the end of the function.
> +	 *
> +	 * The background is that there is a fast path check in
> +	 * dma_fence_is_signaled() which does not use lock protection and can
> +	 * return true *while* the fence callbacks are still executing.
> +	 *
> +	 * This fast path check supposedly cannot be guarded by the lock because
> +	 * of significant performance regressions.
> +	 */
> +	if (unlikely(test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)))
>  		return;
>  
>  	trace_dma_fence_signaled(fence);
> @@ -384,6 +395,9 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence,
>  		INIT_LIST_HEAD(&cur->node);
>  		cur->func(fence, cur);
>  	}
> +
> +	// TODO: we need some barrier here, don't we?

You want a release barrier here, which we don't have currently, but you can use
`smp_mb__before_atomic`.

However note that barriers must be paired, so in order for the added barrier to
do anything, dma_fence_is_signaled lockless fast path would need to have an
acquire barrier, which we also don't have, so you need a `smp_mb()` there after
the flag test.

Best,
Gary

> +	set_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags);
>  }
>  EXPORT_SYMBOL(dma_fence_signal_timestamp_locked);
>