linux-kernel - [PATCH] atomic: Only take lock when the counter drops to zero on UP as well

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Message-Id: <20090411141754.45F7B16080@e179.suse.de>
Date:	Fri, 10 Apr 2009 18:13:57 +0200
From:	Jan Blunck <jblunck@...e.de>
To:	Nick Piggin <npiggin@...e.de>
Cc:	"Paul E. McKenney" <paulmck@...ibm.com>,
	Linux-Kernel Mailinglist <linux-kernel@...r.kernel.org>
Subject: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well

I think it is wrong to unconditionally take the lock before calling
atomic_dec_and_test() in _atomic_dec_and_lock(). This will deadlock in
situation where it is known that the counter will not reach zero (e.g. holding
another reference to the same object) but the lock is already taken.

Signed-off-by: Jan Blunck <jblunck@...e.de>
---
 lib/dec_and_lock.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c
index a65c314..e73822a 100644
--- a/lib/dec_and_lock.c
+++ b/lib/dec_and_lock.c
@@ -19,11 +19,10 @@
  */
 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
 {
-#ifdef CONFIG_SMP
 	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
 	if (atomic_add_unless(atomic, -1, 1))
 		return 0;
-#endif
+
 	/* Otherwise do it the slow way */
 	spin_lock(lock);
 	if (atomic_dec_and_test(atomic))
-- 
1.6.0.2

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