Re: [PATCH 2/8] dma-buf: detach fence ops on signal v2

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

On Tue, 10 Feb 2026 11:01:57 +0100
"Christian König" <ckoenig.leichtzumerken@gmail.com> wrote:

> When neither a release nor a wait backend ops is specified it is possible
> to let the dma_fence live on independently of the module who issued it.
> 
> This makes it possible to unload drivers and only wait for all their
> fences to signal.
> 
> v2: fix typo in comment
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> Reviewed-by: Philipp Stanner <phasta@kernel.org>

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

One nit below.

> ---
>  drivers/dma-buf/dma-fence.c | 16 ++++++++++++----
>  include/linux/dma-fence.h   |  4 ++--
>  2 files changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index de9bf18be3d4..ba02321bef0b 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -371,6 +371,14 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence,
>  				      &fence->flags)))
>  		return;
>  
> +	/*
> +	 * When neither a release nor a wait operation is specified set the ops
> +	 * pointer to NULL to allow the fence structure to become independent
> +	 * from who originally issued it.

I think this deserves some comment in the dma_fence_ops doc, so that
people know what to expect when they implement this interface.

> +	 */
> +	if (!fence->ops->release && !fence->ops->wait)
> +		RCU_INIT_POINTER(fence->ops, NULL);
> +
>  	/* Stash the cb_list before replacing it with the timestamp */
>  	list_replace(&fence->cb_list, &cb_list);
>  
> @@ -537,7 +545,7 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout)
>  	rcu_read_lock();
>  	ops = rcu_dereference(fence->ops);
>  	trace_dma_fence_wait_start(fence);
> -	if (ops->wait) {
> +	if (ops && ops->wait) {
>  		/*
>  		 * Implementing the wait ops is deprecated and not supported for
>  		 * issuer independent fences, so it is ok to use the ops outside
> @@ -602,7 +610,7 @@ void dma_fence_release(struct kref *kref)
>  	}
>  
>  	ops = rcu_dereference(fence->ops);
> -	if (ops->release)
> +	if (ops && ops->release)
>  		ops->release(fence);
>  	else
>  		dma_fence_free(fence);
> @@ -638,7 +646,7 @@ static bool __dma_fence_enable_signaling(struct dma_fence *fence)
>  
>  	rcu_read_lock();
>  	ops = rcu_dereference(fence->ops);
> -	if (!was_set && ops->enable_signaling) {
> +	if (!was_set && ops && ops->enable_signaling) {
>  		trace_dma_fence_enable_signal(fence);
>  
>  		if (!ops->enable_signaling(fence)) {
> @@ -1024,7 +1032,7 @@ void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline)
>  
>  	rcu_read_lock();
>  	ops = rcu_dereference(fence->ops);
> -	if (ops->set_deadline && !dma_fence_is_signaled(fence))
> +	if (ops && ops->set_deadline && !dma_fence_is_signaled(fence))
>  		ops->set_deadline(fence, deadline);
>  	rcu_read_unlock();
>  }
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index 6bf4feb0e01f..e1afbb5909f9 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -472,7 +472,7 @@ dma_fence_is_signaled_locked(struct dma_fence *fence)
>  
>  	rcu_read_lock();
>  	ops = rcu_dereference(fence->ops);
> -	if (ops->signaled && ops->signaled(fence)) {
> +	if (ops && ops->signaled && ops->signaled(fence)) {
>  		rcu_read_unlock();
>  		dma_fence_signal_locked(fence);
>  		return true;
> @@ -508,7 +508,7 @@ dma_fence_is_signaled(struct dma_fence *fence)
>  
>  	rcu_read_lock();
>  	ops = rcu_dereference(fence->ops);
> -	if (ops->signaled && ops->signaled(fence)) {
> +	if (ops && ops->signaled && ops->signaled(fence)) {
>  		rcu_read_unlock();
>  		dma_fence_signal(fence);
>  		return true;