linux-kernel - [PATCH] kref: prefer atomic_inc_not_zero to atomic_add

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Date:	Sat, 10 Oct 2015 12:56:34 +0200
From:	"Jason A. Donenfeld" <Jason@...c4.com>
To:	Dave Airlie <airlied@...hat.com>,
	Thomas Hellstrom <thellstrom@...are.com>,
	linux-kernel@...r.kernel.org
Cc:	"Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH] kref: prefer atomic_inc_not_zero to atomic_add_unless

On most platforms, there exists this ifdef:

 #define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)

This makes this patch functionally useless. However, on PPC, there is
actually an explicit definition of atomic_inc_not_zero with its own
assembly that is slightly more optimized than atomic_add_unless. So,
this patch changes kref to use atomic_inc_not_zero instead, for PPC and
any future platforms that might provide an explicit implementation.

This also puts this usage of kref more in line with a verbatim reading
of the examples in Paul McKenney's paper [1] in the section titled "2.4
Atomic Counting With Check and Release Memory Barrier", which uses
atomic_inc_not_zero.

[1] http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2167.pdf

Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 include/linux/kref.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 484604d..83d1f94 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -166,6 +166,6 @@ static inline int kref_put_mutex(struct kref *kref,
  */
 static inline int __must_check kref_get_unless_zero(struct kref *kref)
 {
-	return atomic_add_unless(&kref->refcount, 1, 0);
+	return atomic_inc_not_zero(&kref->refcount);
 }
 #endif /* _KREF_H_ */
-- 
2.6.0

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