linux-kernel - [PATCH] atomic: Only take lock when the counter drops to zero on UP as well (v2)

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Date:	Thu, 23 Apr 2009 15:51:05 +0200
From:	Jan Blunck <jblunck@...e.de>
To:	linux-kernel@...r.kernel.org
Cc:	npiggin@...e.de, paulmck@...ibm.com, akpm@...ux-foundation.org
Subject: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well (v2)

_atomic_dec_and_lock() should not unconditionally take the lock before
calling atomic_dec_and_test() in the UP case. For consistency reasons it
should behave exactly like in the SMP case.
    
Besides that this works around the problem that with CONFIG_DEBUG_SPINLOCK
this spins in __spin_lock_debug() if the lock is already taken even if the
counter doesn't drop to 0.

Signed-off-by: Jan Blunck <jblunck@...e.de>
---
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))
--
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