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authorAmerigo Wang <amwang@redhat.com>2009-12-15 02:00:21 (GMT)
committerLinus Torvalds <torvalds@linux-foundation.org>2009-12-15 16:53:26 (GMT)
commit29671f22a8b6522db3b126a3fdfb208759ce46e3 (patch)
treec3c434fc07aae30c401fe7662b8e1399b98ffdc2 /lib/rwsem-spinlock.c
parent118d52da1816471ac875bb9f1ee51737e82b1d71 (diff)
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>
Diffstat (limited to 'lib/rwsem-spinlock.c')
-rw-r--r--lib/rwsem-spinlock.c13
1 files changed, 13 insertions, 0 deletions
diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
index 39a7411..ccf95bf 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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