Re: [PATCH] drm/radeon: restore hardware polling in fence_is_signaled to fix performance regression

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] Calling `radeon_fence_activity()` in `.signaled` callback consumes hardware state without waking up wait queues, leading to permanent hangs.
- [Medium] Acquiring an `rw_semaphore` in the `dma_fence_ops.signaled` callback violates wait-context rules and can crash PREEMPT_RT kernels.
--

--- Patch [1]: [PATCH] drm/radeon: restore hardware polling in fence_is_signaled to fix performance regression ---
commit 23682e8220434ea07b863fe650003e08c35ce798
Author: Wang Jiang <jiangwang@kylinos.cn>

drm/radeon: restore hardware polling in fence_is_signaled to fix performance regression

This commit restores hardware polling by calling radeon_fence_activity()
wrapped in down_read_trylock() inside radeon_fence_is_signaled(). This is
intended to fix performance regressions without re-introducing deadlocks.

> diff --git a/drivers/gpu/drm/radeon/radeon_fence.c b/drivers/gpu/drm/radeon/radeon_fence.c
> index 02a40e4750c7d..299865569252d 100644
> --- a/drivers/gpu/drm/radeon/radeon_fence.c
> +++ b/drivers/gpu/drm/radeon/radeon_fence.c
> @@ -360,6 +360,13 @@ static bool radeon_fence_is_signaled(struct dma_fence *f)
>  	if (atomic64_read(&rdev->fence_drv[ring].last_seq) >= seq)
>  		return true;
>  
> +	if (down_read_trylock(&rdev->exclusive_lock)) {
> +		radeon_fence_activity(rdev, ring);

[Severity: High]
Does this consume the hardware state and drop the wake signal?

When a thread calls dma_fence_is_signaled() via polling, it invokes
radeon_fence_is_signaled(), which then calls radeon_fence_activity(). 
The hardware sequence is read, last_seq is advanced, and it returns true.

However, radeon_fence_is_signaled() intentionally drops this return value and
skips wake_up_all(). Later, when the actual hardware interrupt fires,
radeon_fence_process() calls radeon_fence_activity() again.

Because last_seq was already advanced, it sees no new activity
(seq <= last_seq), returns false, and skips wake_up_all().

Could this steal the wake event and cause tasks waiting on fences to hang
indefinitely?

> +		up_read(&rdev->exclusive_lock);

[Severity: Medium]
Is it safe to manipulate an rw_semaphore in the dma_fence_ops.signaled
callback?

The dma_fence_ops.signaled callback can be called from atomic context or
directly from a foreign device's hardirq handler due to cross-device
dependencies.

While down_read_trylock() avoids sleeping on acquire, up_read() still
manipulates the underlying lock state. On PREEMPT_RT, this is an rt_mutex,
and manipulating it from hardirq context violates RT constraints, potentially
leading to a crash.

> +
> +		if (atomic64_read(&rdev->fence_drv[ring].last_seq) >= seq)
> +			return true;
> +	}
>  	return false;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/tencent_BB2A7CAE632BA0AC08D9296E78A15786200A@qq.com?part=1