linux-kernel - [PATCH] lockref: use cmpxchg64 explicitly for lockless updates

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Date:	Thu, 19 Sep 2013 19:06:46 +0100
From:	Will Deacon <will.deacon@....com>
To:	linux-kernel@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, Will Deacon <will.deacon@....com>,
	Waiman Long <Waiman.Long@...com>
Subject: [PATCH] lockref: use cmpxchg64 explicitly for lockless updates

The cmpxchg() function tends not to support 64-bit arguments on 32-bit
architectures. This could be either due to use of unsigned long arguments
(like on ARM) or lack of instruction support (cmpxchgq on x86). However,
these architectures may implement a specific cmpxchg64() function to
provide 64-bit cmpxchg support instead.

Since the lockref code requires a 64-bit cmpxchg and relies on the
architecture selecting ARCH_USE_CMPXCHG_LOCKREF, move to using cmpxchg64
instead of cmpxchg and allow 32-bit architectures to make use of the
lockless lockref implementation.

Cc: Waiman Long <Waiman.Long@...com>
Signed-off-by: Will Deacon <will.deacon@....com>
---

An alternative to this patch is to go through the 32-bit architectures
that implement cmpxchg64, reworking their cmpxchg definitions to switch
to the 64-bit version based on the sizeof the argument.

 lib/lockref.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/lockref.c b/lib/lockref.c
index e2cd2c0..677d036 100644
--- a/lib/lockref.c
+++ b/lib/lockref.c
@@ -14,8 +14,8 @@
 	while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) {  	\
 		struct lockref new = old, prev = old;				\
 		CODE								\
-		old.lock_count = cmpxchg(&lockref->lock_count,			\
-					 old.lock_count, new.lock_count);	\
+		old.lock_count = cmpxchg64(&lockref->lock_count,		\
+					   old.lock_count, new.lock_count);	\
 		if (likely(old.lock_count == prev.lock_count)) {		\
 			SUCCESS;						\
 		}								\
-- 
1.8.2.2

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