dma-buf: detach fence ops on signal v3

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
v3: fix sparse rcu warnings

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>
Link: https://lore.kernel.org/r/20260219160822.1529-3-christian.koenig@amd.com

2 files changed, 16 insertions, 6 deletions

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 076e6e6c75be..3279d82ffa98 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -362,6 +362,7 @@ void __dma_fence_might_wait(void)
 void dma_fence_signal_timestamp_locked(struct dma_fence *fence,
 				      ktime_t timestamp)
 {
+	const struct dma_fence_ops *ops;
 	struct dma_fence_cb *cur, *tmp;
 	struct list_head cb_list;
 
@@ -371,6 +372,15 @@ 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.
+	 */
+	ops = rcu_dereference_protected(fence->ops, true);
+	if (!ops->release && !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 +547,7 @@ 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
 		 * issuers of fences who need their lifetime to be independent
@@ -603,7 +613,7 @@ 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);
@@ -639,7 +649,7 @@ 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)) {
@@ -1025,7 +1035,7 @@ 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 fa3cfe3e98ac..9ff2c4a09cdc 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;