Merge tag 'trace-v4.8' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux...

[cascardo/linux.git] / kernel / locking / rwsem.h

diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index 870ed9a..a699f40 100644 (file)
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -1,14 +1,58 @@
+/*
+ * The owner field of the rw_semaphore structure will be set to
+ * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
+ * the owner field when it unlocks. A reader, on the other hand, will
+ * not touch the owner field when it unlocks.
+ *
+ * In essence, the owner field now has the following 3 states:
+ *  1) 0
+ *     - lock is free or the owner hasn't set the field yet
+ *  2) RWSEM_READER_OWNED
+ *     - lock is currently or previously owned by readers (lock is free
+ *       or not set by owner yet)
+ *  3) Other non-zero value
+ *     - a writer owns the lock
+ */
+#define RWSEM_READER_OWNED     ((struct task_struct *)1UL)
+
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+/*
+ * All writes to owner are protected by WRITE_ONCE() to make sure that
+ * store tearing can't happen as optimistic spinners may read and use
+ * the owner value concurrently without lock. Read from owner, however,
+ * may not need READ_ONCE() as long as the pointer value is only used
+ * for comparison and isn't being dereferenced.
+ */
 static inline void rwsem_set_owner(struct rw_semaphore *sem)
 {
-       sem->owner = current;
+       WRITE_ONCE(sem->owner, current);
 }
 
 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 {
-       sem->owner = NULL;
+       WRITE_ONCE(sem->owner, NULL);
+}
+
+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+       /*
+        * We check the owner value first to make sure that we will only
+        * do a write to the rwsem cacheline when it is really necessary
+        * to minimize cacheline contention.
+        */
+       if (sem->owner != RWSEM_READER_OWNED)
+               WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
+}
+
+static inline bool rwsem_owner_is_writer(struct task_struct *owner)
+{
+       return owner && owner != RWSEM_READER_OWNED;
 }
 
+static inline bool rwsem_owner_is_reader(struct task_struct *owner)
+{
+       return owner == RWSEM_READER_OWNED;
+}
 #else
 static inline void rwsem_set_owner(struct rw_semaphore *sem)
 {
@@ -17,4 +61,8 @@ static inline void rwsem_set_owner(struct rw_semaphore *sem)
 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 {
 }
+
+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+}
 #endif