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 <[email protected]>
> ---
> 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);
> }
>
> /**
