Re: [RFC PATCH] dma-fence: Fix races of fence callbacks versus destructors by locking

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon,  8 Jun 2026 16:24:37 +0200
Philipp Stanner <phasta@kernel.org> wrote:

> @@ -1020,11 +1024,20 @@ EXPORT_SYMBOL(dma_fence_wait_any_timeout);
>  void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline)
>  {
>  	const struct dma_fence_ops *ops;
> +	unsigned long flags;
>  
>  	rcu_read_lock();
>  	ops = rcu_dereference(fence->ops);
> -	if (ops && ops->set_deadline && !dma_fence_is_signaled(fence))
> +	if (!ops || !ops->set_deadline) {
> +		rcu_read_unlock();
> +		return;
> +	}
> +
> +	dma_fence_lock_irqsave(fence, flags);
> +	if (!dma_fence_is_signaled_locked(fence))
>  		ops->set_deadline(fence, deadline);

You can't take the fence lock around ->set_deadline(), otherwise you'll
deadlock here [1] or here [2].

> +
> +	dma_fence_unlock_irqrestore(fence, flags);
>  	rcu_read_unlock();
>  }


[1]https://elixir.bootlin.com/linux/v7.0.11/source/drivers/dma-buf/sw_sync.c#L182
[2]https://elixir.bootlin.com/linux/v7.0.11/source/drivers/gpu/drm/msm/msm_fence.c#L139