dma-buf: protected fence ops by RCU v8

The fence ops of a dma_fence currently need to life as long as the
dma_fence is alive. This means that the module which originally issued
a dma_fence can't unload unless all fences are freed up.

As first step to solve this issue protect the fence ops by RCU.

While it is counter intuitive to protect a constant function pointer table
by RCU it allows modules to wait for an RCU grace period before they
unload, to make sure that nobody is executing their functions any more.

This patch has not much functional change, but only adds the RCU
handling for the static checker to test.

v2: make one the now duplicated lockdep warnings a comment instead.
v3: Add more documentation to ->wait and ->release callback.
v4: fix typo in documentation
v5: rebased on drm-tip
v6: improve code comments
v7: improve commit message and code comments
v8: fix sparse rcu warnings

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/r/20260219160822.1529-2-christian.koenig@amd.com

1 files changed, 28 insertions, 5 deletions

diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index 9c4d25289239..fa3cfe3e98ac 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -67,7 +67,7 @@ struct seq_file;
  */
 struct dma_fence {
 	spinlock_t *lock;
-	const struct dma_fence_ops *ops;
+	const struct dma_fence_ops __rcu *ops;
 	/*
 	 * We clear the callback list on kref_put so that by the time we
 	 * release the fence it is unused. No one should be adding to the
@@ -220,6 +220,10 @@ struct dma_fence_ops {
 	 * timed out. Can also return other error values on custom implementations,
 	 * which should be treated as if the fence is signaled. For example a hardware
 	 * lockup could be reported like that.
+	 *
+	 * Implementing this callback prevents the fence from detaching after
+	 * signaling and so it is necessary for the module providing the
+	 * dma_fence_ops to stay loaded as long as the dma_fence exists.
 	 */
 	signed long (*wait)(struct dma_fence *fence,
 			    bool intr, signed long timeout);
@@ -231,6 +235,13 @@ struct dma_fence_ops {
 	 * Can be called from irq context.  This callback is optional. If it is
 	 * NULL, then dma_fence_free() is instead called as the default
 	 * implementation.
+	 *
+	 * Implementing this callback prevents the fence from detaching after
+	 * signaling and so it is necessary for the module providing the
+	 * dma_fence_ops to stay loaded as long as the dma_fence exists.
+	 *
+	 * If the callback is implemented the memory backing the dma_fence
+	 * object must be freed RCU safe.
 	 */
 	void (*release)(struct dma_fence *fence);
 
@@ -454,13 +465,19 @@ dma_fence_test_signaled_flag(struct dma_fence *fence)
 static inline bool
 dma_fence_is_signaled_locked(struct dma_fence *fence)
 {
+	const struct dma_fence_ops *ops;
+
 	if (dma_fence_test_signaled_flag(fence))
 		return true;
 
-	if (fence->ops->signaled && fence->ops->signaled(fence)) {
+	rcu_read_lock();
+	ops = rcu_dereference(fence->ops);
+	if (ops->signaled && ops->signaled(fence)) {
+		rcu_read_unlock();
 		dma_fence_signal_locked(fence);
 		return true;
 	}
+	rcu_read_unlock();
 
 	return false;
 }
@@ -484,13 +501,19 @@ dma_fence_is_signaled_locked(struct dma_fence *fence)
 static inline bool
 dma_fence_is_signaled(struct dma_fence *fence)
 {
+	const struct dma_fence_ops *ops;
+
 	if (dma_fence_test_signaled_flag(fence))
 		return true;
 
-	if (fence->ops->signaled && fence->ops->signaled(fence)) {
+	rcu_read_lock();
+	ops = rcu_dereference(fence->ops);
+	if (ops->signaled && ops->signaled(fence)) {
+		rcu_read_unlock();
 		dma_fence_signal(fence);
 		return true;
 	}
+	rcu_read_unlock();
 
 	return false;
 }
@@ -695,7 +718,7 @@ extern const struct dma_fence_ops dma_fence_chain_ops;
  */
 static inline bool dma_fence_is_array(struct dma_fence *fence)
 {
-	return fence->ops == &dma_fence_array_ops;
+	return rcu_access_pointer(fence->ops) == &dma_fence_array_ops;
 }
 
 /**
@@ -706,7 +729,7 @@ static inline bool dma_fence_is_array(struct dma_fence *fence)
  */
 static inline bool dma_fence_is_chain(struct dma_fence *fence)
 {
-	return fence->ops == &dma_fence_chain_ops;
+	return rcu_access_pointer(fence->ops) == &dma_fence_chain_ops;
 }
 
 /**