From 29671f22a8b6522db3b126a3fdfb208759ce46e3 Mon Sep 17 00:00:00 2001 From: Amerigo Wang Date: Mon, 14 Dec 2009 18:00:21 -0800 Subject: 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 Cc: Ben Woodard Cc: David Howells Signed-off-by: WANG Cong Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- lib/rwsem-spinlock.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'lib/rwsem-spinlock.c') diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c index 39a74110f55b..ccf95bff7984 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 */ -- cgit v1.2.3