linux-kernel - [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear

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Message-Id: <20070723160608.22137.14790.sendpatchset@cselinux1.cse.iitk.ac.in>
Date:	Mon, 23 Jul 2007 21:36:08 +0530
From:	Satyam Sharma <ssatyam@....iitk.ac.in>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear_bit() definitions

From: Satyam Sharma <ssatyam@....iitk.ac.in>

[8/8] i386: bitops: smp_mb__{before, after}_clear_bit() definitions

>From Documentation/atomic_ops.txt, those archs that require explicit
memory barriers around clear_bit() must also implement these two interfaces.
However, for i386, clear_bit() is a strict, locked, atomic and
un-reorderable operation and includes an implicit memory barrier already.

But these two functions have been wrongly defined as "barrier()" which is
a pointless _compiler optimization_ barrier, and only serves to make gcc
not do legitimate optimizations that it could have otherwise done.

So let's make these proper no-ops, because that's exactly what we require
these to be on the i386 platform.

Signed-off-by: Satyam Sharma <ssatyam@....iitk.ac.in>
Cc: David Howells <dhowells@...hat.com>
Cc: Nick Piggin <nickpiggin@...oo.com.au>

---

[ A similar optimization needs to be done in the atomic.h also.
  Will submit that patch shortly. ]

 include/asm-i386/bitops.h |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/asm-i386/bitops.h b/include/asm-i386/bitops.h
index 4f1fda5..42999eb 100644
--- a/include/asm-i386/bitops.h
+++ b/include/asm-i386/bitops.h
@@ -106,8 +106,8 @@ static inline void __clear_bit(int nr, unsigned long *addr)
  * Bit operations are already serializing on x86.
  * These must still be defined here for API completeness.
  */
-#define smp_mb__before_clear_bit()	barrier()
-#define smp_mb__after_clear_bit()	barrier()
+#define smp_mb__before_clear_bit()	do {} while (0)
+#define smp_mb__after_clear_bit()	do {} while (0)
 
 /**
  * __change_bit - Toggle a bit in memory
-
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