atomic: only take lock when the counter drops to zero on UP as well
Jan Blunck [Tue, 16 Jun 2009 22:33:33 +0000 (15:33 -0700)]
_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

index a65c314..e73822a 100644 (file)
  */
 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))