Commit fb6a44f3 authored May 17, 2016 by Waiman Long Committed by Ingo Molnar Jun 08, 2016

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>

parent 19c5d690

kernel/locking/rwsem.h

+10 −3

Original line number	Diff line number	Diff line
		@@ -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)

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