reservation: add suppport for read-only access using rcu

This adds some extra functions to deal with rcu.

reservation_object_get_fences_rcu() will obtain the list of shared
and exclusive fences without obtaining the ww_mutex.

reservation_object_wait_timeout_rcu() will wait on all fences of the
reservation_object, without obtaining the ww_mutex.

reservation_object_test_signaled_rcu() will test if all fences of the
reservation_object are signaled without using the ww_mutex.

reservation_object_get_excl and reservation_object_get_list require
the reservation object to be held, updating requires
write_seqcount_begin/end. If only the exclusive fence is needed,
rcu_dereference followed by fence_get_rcu can be used, if the shared
fences are needed it's recommended to use the supplied functions.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-By: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 53 insertions, 16 deletions

diff --git a/include/linux/fence.h b/include/linux/fence.h
index b935cc65012..d174585b874 100644
--- a/include/linux/fence.h
+++ b/include/linux/fence.h
@@ -28,6 +28,7 @@
 #include <linux/kref.h>
 #include <linux/sched.h>
 #include <linux/printk.h>
+#include <linux/rcupdate.h>
 
 struct fence;
 struct fence_ops;
@@ -37,6 +38,7 @@ struct fence_cb;
  * struct fence - software synchronization primitive
  * @refcount: refcount for this fence
  * @ops: fence_ops associated with this fence
+ * @rcu: used for releasing fence with kfree_rcu
  * @cb_list: list of all callbacks to call
  * @lock: spin_lock_irqsave used for locking
  * @context: execution context this fence belongs to, returned by
@@ -70,6 +72,7 @@ struct fence_cb;
 struct fence {
 	struct kref refcount;
 	const struct fence_ops *ops;
+	struct rcu_head rcu;
 	struct list_head cb_list;
 	spinlock_t *lock;
 	unsigned context, seqno;
@@ -192,6 +195,20 @@ static inline struct fence *fence_get(struct fence *fence)
 }
 
 /**
+ * fence_get_rcu - get a fence from a reservation_object_list with rcu read lock
+ * @fence:	[in]	fence to increase refcount of
+ *
+ * Function returns NULL if no refcount could be obtained, or the fence.
+ */
+static inline struct fence *fence_get_rcu(struct fence *fence)
+{
+	if (kref_get_unless_zero(&fence->refcount))
+		return fence;
+	else
+		return NULL;
+}
+
+/**
  * fence_put - decreases refcount of the fence
  * @fence:	[in]	fence to reduce refcount of
  */
diff --git a/include/linux/reservation.h b/include/linux/reservation.h
index 2affe67dea6..5a0b64cf68b 100644
--- a/include/linux/reservation.h
+++ b/include/linux/reservation.h
@@ -42,22 +42,29 @@
 #include <linux/ww_mutex.h>
 #include <linux/fence.h>
 #include <linux/slab.h>
+#include <linux/seqlock.h>
+#include <linux/rcupdate.h>
 
 extern struct ww_class reservation_ww_class;
+extern struct lock_class_key reservation_seqcount_class;
+extern const char reservation_seqcount_string[];
 
 struct reservation_object_list {
+	struct rcu_head rcu;
 	u32 shared_count, shared_max;
-	struct fence *shared[];
+	struct fence __rcu *shared[];
 };
 
 struct reservation_object {
 	struct ww_mutex lock;
+	seqcount_t seq;
 
-	struct fence *fence_excl;
-	struct reservation_object_list *fence;
+	struct fence __rcu *fence_excl;
+	struct reservation_object_list __rcu *fence;
 	struct reservation_object_list *staged;
 };
 
+#define reservation_object_held(obj) lockdep_is_held(&(obj)->lock.base)
 #define reservation_object_assert_held(obj) \
 	lockdep_assert_held(&(obj)->lock.base)
 
@@ -66,8 +73,9 @@ reservation_object_init(struct reservation_object *obj)
 {
 	ww_mutex_init(&obj->lock, &reservation_ww_class);
 
-	obj->fence_excl = NULL;
-	obj->fence = NULL;
+	__seqcount_init(&obj->seq, reservation_seqcount_string, &reservation_seqcount_class);
+	RCU_INIT_POINTER(obj->fence, NULL);
+	RCU_INIT_POINTER(obj->fence_excl, NULL);
 	obj->staged = NULL;
 }
 
@@ -76,18 +84,20 @@ reservation_object_fini(struct reservation_object *obj)
 {
 	int i;
 	struct reservation_object_list *fobj;
+	struct fence *excl;
 
 	/*
 	 * This object should be dead and all references must have
-	 * been released to it.
+	 * been released to it, so no need to be protected with rcu.
 	 */
-	if (obj->fence_excl)
-		fence_put(obj->fence_excl);
+	excl = rcu_dereference_protected(obj->fence_excl, 1);
+	if (excl)
+		fence_put(excl);
 
-	fobj = obj->fence;
+	fobj = rcu_dereference_protected(obj->fence, 1);
 	if (fobj) {
 		for (i = 0; i < fobj->shared_count; ++i)
-			fence_put(fobj->shared[i]);
+			fence_put(rcu_dereference_protected(fobj->shared[i], 1));
 
 		kfree(fobj);
 	}
@@ -99,17 +109,15 @@ reservation_object_fini(struct reservation_object *obj)
 static inline struct reservation_object_list *
 reservation_object_get_list(struct reservation_object *obj)
 {
-	reservation_object_assert_held(obj);
-
-	return obj->fence;
+	return rcu_dereference_protected(obj->fence,
+					 reservation_object_held(obj));
 }
 
 static inline struct fence *
 reservation_object_get_excl(struct reservation_object *obj)
 {
-	reservation_object_assert_held(obj);
-
-	return obj->fence_excl;
+	return rcu_dereference_protected(obj->fence_excl,
+					 reservation_object_held(obj));
 }
 
 int reservation_object_reserve_shared(struct reservation_object *obj);
@@ -119,4 +127,16 @@ void reservation_object_add_shared_fence(struct reservation_object *obj,
 void reservation_object_add_excl_fence(struct reservation_object *obj,
 				       struct fence *fence);
 
+int reservation_object_get_fences_rcu(struct reservation_object *obj,
+				      struct fence **pfence_excl,
+				      unsigned *pshared_count,
+				      struct fence ***pshared);
+
+long reservation_object_wait_timeout_rcu(struct reservation_object *obj,
+					 bool wait_all, bool intr,
+					 unsigned long timeout);
+
+bool reservation_object_test_signaled_rcu(struct reservation_object *obj,
+					  bool test_all);
+
 #endif /* _LINUX_RESERVATION_H */