[PATCH] atomic: dec_and_lock use atomic primitives

Convert atomic_dec_and_lock to use new atomic primitives.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 6 insertions, 43 deletions

diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c
index 305a9663aee..a65c3145554 100644
--- a/lib/dec_and_lock.c
+++ b/lib/dec_and_lock.c
@@ -1,47 +1,11 @@
 #include <linux/module.h>
 #include <linux/spinlock.h>
 #include <asm/atomic.h>
-#include <asm/system.h>
 
-#ifdef __HAVE_ARCH_CMPXCHG
 /*
  * This is an implementation of the notion of "decrement a
  * reference count, and return locked if it decremented to zero".
  *
- * This implementation can be used on any architecture that
- * has a cmpxchg, and where atomic->value is an int holding
- * the value of the atomic (i.e. the high bits aren't used
- * for a lock or anything like that).
- */
-int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
-{
-        int counter;
-        int newcount;
-
-        for (;;) {
-                counter = atomic_read(atomic);
-                newcount = counter - 1;
-                if (!newcount)
-                        break;          /* do it the slow way */
-
-                newcount = cmpxchg(&atomic->counter, counter, newcount);
-                if (newcount == counter)
-                        return 0;
-        }
-
-        spin_lock(lock);
-        if (atomic_dec_and_test(atomic))
-                return 1;
-        spin_unlock(lock);
-        return 0;
-}
-#else
-/*
- * This is an architecture-neutral, but slow,
- * implementation of the notion of "decrement
- * a reference count, and return locked if it
- * decremented to zero".
- *
  * NOTE NOTE NOTE! This is _not_ equivalent to
  *
  *      if (atomic_dec_and_test(&atomic)) {
@@ -52,21 +16,20 @@ int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
  *
  * because the spin-lock and the decrement must be
  * "atomic".
- *
- * This slow version gets the spinlock unconditionally,
- * and releases it if it isn't needed. Architectures
- * are encouraged to come up with better approaches,
- * this is trivially done efficiently using a load-locked
- * store-conditional approach, for example.
  */
 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))
                 return 1;
         spin_unlock(lock);
         return 0;
 }
-#endif
 
 EXPORT_SYMBOL(_atomic_dec_and_lock);