On 7/30/26 03:56, [email protected] wrote:
> From: Wang Jiang <[email protected]>
>
> Commit 9eb00b5f5697b ("drm/radeon: delete radeon_fence_process in
> is_signaled, no deadlock") removed the hardware status check from
> radeon_fence_is_signaled() to fix a self-deadlock caused by
> wake_up_all(&rdev->fence_queue) being called with the fence queue
> lock held.
>
> However, removing the hardware poll entirely causes significant
> performance regression (e.g. glxgears FPS drop) because the signaled
> check becomes purely passive — it only reads the cached last_seq
> without probing the GPU, so completed GPU work is not detected in
> time, causing unnecessary CPU stalls in sync-heavy workloads.
>
> Fix this by calling radeon_fence_read() directly in
> radeon_fence_is_signaled() to poll the hardware fence counter
> without updating last_seq or calling wake_up_all(). This restores
> timely fence detection while avoiding the original deadlock, since
> last_seq update and waiter wakeup remain solely in the interrupt
> handler's responsibility.
>
> Fixes: 9eb00b5f5697b ("drm/radeon: delete radeon_fence_process in
> is_signaled, no deadlock")
> Signed-off-by: Wang Jiang <[email protected]>
> ---
> drivers/gpu/drm/radeon/radeon_fence.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/drivers/gpu/drm/radeon/radeon_fence.c
> b/drivers/gpu/drm/radeon/radeon_fence.c
> index 02a40e4750c7..047ec5a1d194 100644
> --- a/drivers/gpu/drm/radeon/radeon_fence.c
> +++ b/drivers/gpu/drm/radeon/radeon_fence.c
> @@ -360,6 +360,18 @@ static bool radeon_fence_is_signaled(struct dma_fence *f)
> if (atomic64_read(&rdev->fence_drv[ring].last_seq) >= seq)
> return true;
>
> + /*
> + * Poll hardware directly without updating last_seq.
> + * This allows immediate detection of signaled fences (performance)
> + * while leaving last_seq update and wake_up_all() to the interrupt
> + * handler, avoiding:
> + * 1. Deadlock from calling wake_up_all() with fence lock held
> + * 2. Wake event stealing by advancing last_seq without waking waiters
> + * 3. rw_semaphore usage in atomic/irq context (unsafe on PREEMPT_RT)
> + */
> + if (radeon_fence_read(rdev, ring) >= (u32)seq)
> + return true;
> +
That check is not correct, you need to take wrap around into account.
Try using __dma_fence_is_later() instead.
Regards,
Christian.
> return false;
> }
>
