linux-kernel - [PATCH] Add ACCESS_ONCE() to avoid compiler splitting assignments

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Date:	Tue, 15 Jan 2013 10:19:36 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	lttng-dev@...ts.lttng.org, linux-kernel@...r.kernel.org,
	rp@...s.cs.pdx.edu
Cc:	mathieu.desnoyers@...icios.com, laijs@...fujitsu.com,
	shemminger@...tta.com, stern@...land.harvard.edu,
	khlebnikov@...nvz.org
Subject: [PATCH] Add ACCESS_ONCE() to avoid compiler splitting assignments

As noted by Konstantin Khlebnikov, gcc can split assignment of
constants to long variables (https://lkml.org/lkml/2013/1/15/141),
though assignment of NULL (0) is OK.  Assuming that a gcc bug is
fixed (http://gcc.gnu.org/bugzilla/attachment.cgi?id=29169&action=diff
has a patch), making the store be volatile keeps gcc from splitting.

This commit therefore applies ACCESS_ONCE() to CMM_STORE_SHARED(),
which is the underlying primitive used by rcu_assign_pointer().

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/urcu/system.h b/urcu/system.h
index 2a45f22..7a1887e 100644
--- a/urcu/system.h
+++ b/urcu/system.h
@@ -49,7 +49,7 @@
  */
 #define CMM_STORE_SHARED(x, v)		\
 	({				\
-		__typeof__(x) _v = _CMM_STORE_SHARED(x, v);	\
+		__typeof__(x) CMM_ACCESS_ONCE(_v) = _CMM_STORE_SHARED(x, v);	\
 		cmm_smp_wmc();		\
 		_v;			\
 	})

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