atomic: only take lock when the counter drops to zero on UP as well

_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@suse.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Cc: Valerie Aurora <vaurora@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c
index a65c314555416d9f1ea262455d1da313e1959902..e73822aa6e9a68131df955c0a6025c7aa5556fc6 100644 (file)
--- 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))