linux-kernel - [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE

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Date:	Tue, 17 May 2016 21:26:20 -0400
From:	Waiman Long <Waiman.Long@....com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Waiman Long <Waiman.Long@....com>
Subject: [PATCH v4 2/5] 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@....com>
---
 kernel/locking/rwsem.h |   13 ++++++++++---
 1 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index 8f43ba2..a699f40 100644
--- a/kernel/locking/rwsem.h
+++ b/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)
-- 
1.7.1