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>

include/linux/rwsem-spinlock.h

@@ -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

@@ -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
  */