[PATCH 4/4] convert dec_and_lock

Nick Piggin
2005-09-24T02:55:35Z

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4/4

This also contains a small optimisation on UP machines
where a spinlock is a noop.

--
SUSE Labs, Novell Inc.

[-- Attachment #2: atomic-dec_and_lock-use-atomic-primitives.patch --]
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Index: linux-2.6/lib/dec_and_lock.c
===================================================================
--- linux-2.6.orig/lib/dec_and_lock.c
+++ linux-2.6/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 *atomi
*
* 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);