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rwsem: fix rwsem_is_locked() bugs

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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().

[[email protected]: simplify code]
Reported-by: Brian Behlendorf <[email protected]>
Cc: Ben Woodard <[email protected]>
Cc: David Howells <[email protected]>
Signed-off-by: WANG Cong <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Amerigo Wang 2009-12-14 18:00:21 -08:00 • committed by Linus Torvalds
commit 29671f22a8
2 changed files with 14 additions and 5 deletions
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13
lib/rwsem-spinlock.c
View file

@ -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
*/