Commit 29671f22 authored Dec 14, 2009 by Amerigo Wang Committed by Linus Torvalds Dec 15, 2009

rwsem: fix rwsem_is_locked() bugs



rwsem_is_locked() tests ->activity without locks, so we should always keep
->activity consistent.  However, the code in __rwsem_do_wake() breaks this
rule, it updates ->activity after _all_ readers waken up, this may give
some reader a wrong ->activity value, thus cause rwsem_is_locked() behaves
wrong.

Quote from Andrew:

"
- we have one or more processes sleeping in down_read(), waiting for access.

- we wake one or more processes up without altering ->activity

- they start to run and they do rwsem_is_locked().  This incorrectly
  returns "false", because the waker process is still crunching away in
  __rwsem_do_wake().

- the waker now alters ->activity, but it was too late.
"

So we need get a spinlock to protect this.  And rwsem_is_locked() should
not block, thus we use spin_trylock_irqsave().

[akpm@linux-foundation.org: simplify code]
Reported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Cc: Ben Woodard <bwoodard@llnl.gov>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 118d52da

include/linux/rwsem-spinlock.h

+1 −5

Original line number	Diff line number	Diff line
		@@ -68,11 +68,7 @@ extern int __down_write_trylock(struct rw_semaphore *sem);
		extern void __up_read(struct rw_semaphore *sem);
		extern void __up_write(struct rw_semaphore *sem);
		extern void __downgrade_write(struct rw_semaphore *sem);

		static inline int rwsem_is_locked(struct rw_semaphore *sem)
		{
		return (sem->activity != 0);
		}
		extern int rwsem_is_locked(struct rw_semaphore *sem);

		#endif /* __KERNEL__ */
		#endif /* _LINUX_RWSEM_SPINLOCK_H */

lib/rwsem-spinlock.c

+13 −0

Original line number	Diff line number	Diff line
		@@ -17,6 +17,19 @@ struct rwsem_waiter {
		#define RWSEM_WAITING_FOR_WRITE 0x00000002
		};

		int rwsem_is_locked(struct rw_semaphore *sem)
		{
		int ret = 1;
		unsigned long flags;

		if (spin_trylock_irqsave(&sem->wait_lock, flags)) {
		ret = (sem->activity != 0);
		spin_unlock_irqrestore(&sem->wait_lock, flags);
		}
		return ret;
		}
		EXPORT_SYMBOL(rwsem_is_locked);

		/*
		* initialise the semaphore
		*/