[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>

[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>
Nick Piggin · Linus Torvalds
1 parent 8382bf2e72
Showing 1 changed file with 6 additions and 43 deletions Side-by-side Diff
lib/dec_and_lock.c
 #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)) {
  
  
@@ -52,22 +16,21 @@
  *
  * 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);
1	1	#include <linux/module.h>
2	2	#include <linux/spinlock.h>
3	3	#include <asm/atomic.h>
4		-#include <asm/system.h>
5	4
6		-#ifdef __HAVE_ARCH_CMPXCHG
7	5	/*
8	6	* This is an implementation of the notion of "decrement a
9	7	* reference count, and return locked if it decremented to zero".
10	8	*
11		- * This implementation can be used on any architecture that
12		- * has a cmpxchg, and where atomic->value is an int holding
13		- * the value of the atomic (i.e. the high bits aren't used
14		- * for a lock or anything like that).
15		- */
16		-int _atomic_dec_and_lock(atomic_t atomic, spinlock_t lock)
17		-{
18		- int counter;
19		- int newcount;
20		-
21		- for (;;) {
22		- counter = atomic_read(atomic);
23		- newcount = counter - 1;
24		- if (!newcount)
25		- break; /* do it the slow way */
26		-
27		- newcount = cmpxchg(&atomic->counter, counter, newcount);
28		- if (newcount == counter)
29		- return 0;
30		- }
31		-
32		- spin_lock(lock);
33		- if (atomic_dec_and_test(atomic))
34		- return 1;
35		- spin_unlock(lock);
36		- return 0;
37		-}
38		-#else
39		-/*
40		- * This is an architecture-neutral, but slow,
41		- * implementation of the notion of "decrement
42		- * a reference count, and return locked if it
43		- * decremented to zero".
44		- *
45	9	* NOTE NOTE NOTE! This is _not_ equivalent to
46	10	*
47	11	* if (atomic_dec_and_test(&atomic)) {
48	12
49	13
...	...	@@ -52,22 +16,21 @@
52	16	*
53	17	* because the spin-lock and the decrement must be
54	18	* "atomic".
55		- *
56		- * This slow version gets the spinlock unconditionally,
57		- * and releases it if it isn't needed. Architectures
58		- * are encouraged to come up with better approaches,
59		- * this is trivially done efficiently using a load-locked
60		- * store-conditional approach, for example.
61	19	*/
62	20	int _atomic_dec_and_lock(atomic_t atomic, spinlock_t lock)
63	21	{
	22	+#ifdef CONFIG_SMP
	23	+ /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
	24	+ if (atomic_add_unless(atomic, -1, 1))
	25	+ return 0;
	26	+#endif
	27	+ /* Otherwise do it the slow way */
64	28	spin_lock(lock);
65	29	if (atomic_dec_and_test(atomic))
66	30	return 1;
67	31	spin_unlock(lock);
68	32	return 0;
69	33	}
70		-#endif
71	34
72	35	EXPORT_SYMBOL(_atomic_dec_and_lock);