Fwd: [PATCH] kref: prefer atomic_inc_not_zero to atomic_add_unless

[Thomas Hellstrom <thellstrom@vmware.com>]

By request forwarded patch

This is also
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>

/Thomas


-------- Forwarded Message --------
Subject: 	[PATCH] kref: prefer atomic_inc_not_zero to atomic_add_unless
Date: 	Sat, 10 Oct 2015 12:56:34 +0200
From: 	Jason A. Donenfeld <Jason@zx2c4.com>
To: 	Dave Airlie <airlied@redhat.com>, Thomas Hellstrom
<thellstrom@vmware.com>, linux-kernel@vger.kernel.org
CC: 	Jason A. Donenfeld <Jason@zx2c4.com>



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] https://urldefense.proofpoint.com/v2/url?u=http-3A__open-2Dstd.org_jtc1_sc22_wg21_docs_papers_2007_n2167.pdf&d=BQIBAg&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=vpukPkBtpoNQp2IUKuFviOmPNYWVKmen3Jeeu55zmEA&m=z5Nd9sYiJMKiphNjyZp6XT5CbayXMBlcb903f260pDY&s=HEHX3CuXRs2GRRQWuC4Vef6iJMwdilKVRkiZgJpjEpA&e= 

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.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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