[PATCH] dma-buf: Introduce fence_get_rcu_safe()

[Chris Wilson]

This variant of fence_get_rcu() takes an RCU protected pointer to a
fence and carefully returns a reference to the fence ensuring that it is
not reallocated as it does. This is required when mixing fences and
SLAB_DESTROY_BY_RCU - although it serves a more pedagogical function atm

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Sumit Semwal <sumit.semwal at linaro.org>
---
 include/linux/fence.h | 51 ++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 46 insertions(+), 5 deletions(-)

diff --git a/include/linux/fence.h b/include/linux/fence.h
index 5f50ab273c38..e93f7e70d0fe 100644
--- a/include/linux/fence.h
+++ b/include/linux/fence.h
@@ -187,6 +187,16 @@ void fence_release(struct kref *kref);
 void fence_free(struct fence *fence);
 
 /**
+ * fence_put - decreases refcount of the fence
+ * @fence:	[in]	fence to reduce refcount of
+ */
+static inline void fence_put(struct fence *fence)
+{
+	if (fence)
+		kref_put(&fence->refcount, fence_release);
+}
+
+/**
  * fence_get - increases refcount of the fence
  * @fence:	[in]	fence to increase refcount of
  *
@@ -214,13 +224,44 @@ static inline struct fence *fence_get_rcu(struct fence *fence)
 }
 
 /**
- * fence_put - decreases refcount of the fence
- * @fence:	[in]	fence to reduce refcount of
+ * fence_get_rcu_safe  - acquire a reference to an RCU tracked fence
+ * @fence:	[in]	pointer to fence to increase refcount of
+ *
+ * Function returns NULL if no refcount could be obtained, or the fence.
+ * This function handles acquiring a reference to a fence that may be
+ * reallocated within the RCU grace period (such as with SLAB_DESTROY_BY_RCU),
+ * so long as the caller is using RCU on the pointer to the fence.
+ *
+ * The caller is required to hold the RCU read lock.
  */
-static inline void fence_put(struct fence *fence)
+static inline struct fence *fence_get_rcu_safe(struct fence * __rcu *fencep)
 {
-	if (fence)
-		kref_put(&fence->refcount, fence_release);
+	do {
+		struct fence *fence;
+
+		fence = rcu_dereference(*fencep);
+		if (!fence || !fence_get_rcu(fence))
+			return NULL;
+
+		/* The atomic_inc_not_zero() inside fence_get_rcu()
+		 * provides a full memory barrier upon success (such as now).
+		 * This is paired with the write barrier from assigning
+		 * to the __rcu protected fence pointer so that if that
+		 * pointer still matches the current fence, we know we
+		 * have successfully acquire a reference to it. If it no
+		 * longer matches, we are holding a reference to some other
+		 * reallocated pointer. This is possible if the allocator
+		 * is using a freelist like SLAB_DESTROY_BY_RCU where the
+		 * fence remains valid for the RCU grace period, but it
+		 * may be reallocated. When using such allocators, we are
+		 * responsible for ensuring the reference we get is to
+		 * the right fence, as below.
+		 */
+		if (fence == rcu_access_pointer(*fencep))
+			return fence;
+
+		fence_put(fence);
+	} while (1);
 }
 
 int fence_signal(struct fence *fence);
-- 
2.8.1