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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: lttng-dev@lists.lttng.org, linux-kernel@vger.kernel.org,
	rp@svcs.cs.pdx.edu
Cc: mathieu.desnoyers@efficios.com, laijs@cn.fujitsu.com,
	shemminger@vyatta.com, stern@rowland.harvard.edu,
	khlebnikov@openvz.org
Subject: [PATCH] Add ACCESS_ONCE() to avoid compiler splitting assignments
Date: Tue, 15 Jan 2013 10:19:36 -0800	[thread overview]
Message-ID: <20130115181936.GA30319@linux.vnet.ibm.com> (raw)

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@linux.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;			\
 	})

             reply	other threads:[~2013-01-15 18:19 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-01-15 18:19 Paul E. McKenney [this message]
2013-01-15 23:56 ` [lttng-dev] [PATCH] Add ACCESS_ONCE() to avoid compiler splitting assignments Mathieu Desnoyers
2013-01-16 12:50   ` Mathieu Desnoyers
2013-01-19 19:17     ` Paul E. McKenney
2013-01-20 20:51       ` Mathieu Desnoyers
2013-01-25 13:53         ` Paul E. McKenney

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