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>

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))