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Subject[Patch v5] rwsem: fix rwsem_is_locked() bugs

v4 -> v5:
- Uninline rwsem_is_locked().
- Use spin_trylock_irqsave instead of spin_trylock_irq().

Thanks to Andrew and David.

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().

(David, if you think it's fine, can you Ack on this version?
Thanks!)

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>

---
diff --git a/include/linux/rwsem-spinlock.h b/include/linux/rwsem-spinlock.h
index 6c3c0f6..571be3c 100644
--- a/include/linux/rwsem-spinlock.h
+++ b/include/linux/rwsem-spinlock.h
@@ -68,11 +68,9 @@ 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);
+extern int __rwsem_is_locked(struct rw_semaphore *sem);

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

#endif /* __KERNEL__ */
#endif /* _LINUX_RWSEM_SPINLOCK_H */
diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
index 9df3ca5..7014306 100644
--- a/lib/rwsem-spinlock.c
+++ b/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
*/

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