locking/rwsem: Protect all writes to owner by WRITE_ONCE()

Without using WRITE_ONCE(), the compiler can potentially break a
write into multiple smaller ones (store tearing). So a read from the
same data by another task concurrently may return a partial result.
This can result in a kernel crash if the data is a memory address
that is being dereferenced.

This patch changes all write to rwsem->owner to use WRITE_ONCE()
to make sure that store tearing will not happen. READ_ONCE() may
not be needed for rwsem->owner as long as the value is only used for
comparison and not dereferencing.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1463534783-38814-3-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/rwsem.h

@@ -16,14 +16,21 @@
 #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)
@@ -34,7 +41,7 @@ static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
 	 * to minimize cacheline contention.
 	 */
 	if (sem->owner != RWSEM_READER_OWNED)
-		sem->owner = RWSEM_READER_OWNED;
+		WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
 }
 
 static inline bool rwsem_owner_is_writer(struct task_struct *owner)