netdev - [PATCH net-next] net: relax rtnl

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Date:	Mon, 04 Oct 2010 11:00:12 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	hadi@...erus.ca, netdev <netdev@...r.kernel.org>,
	Jarek Poplawski <jarkao2@...il.com>
Subject: [PATCH net-next] net: relax rtnl_dereference()

Le lundi 04 octobre 2010 à 10:42 +0200, Eric Dumazet a écrit :

> BTW, rtnl_dereference() should be changed to use
> rcu_dereference_protected() instead of rcu_dereference_check() :
> If RTBL is held, there is no need to force a barrier.
> 

[PATCH net-next] net: relax rtnl_dereference()

rtnl_dereference() is used in contexts where RTNL is held, to fetch an
RCU protected pointer.
 
Updates to this pointer are prevented by RTNL, so we dont need
smp_read_barrier_depends() and the ACCESS_ONCE() provided in
rcu_dereference_check().

rtnl_dereference() is mainly a macro to document the locking invariant.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 include/linux/rtnetlink.h |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/linux/rtnetlink.h b/include/linux/rtnetlink.h
index 68c436b..d3c4efa 100644
--- a/include/linux/rtnetlink.h
+++ b/include/linux/rtnetlink.h
@@ -754,20 +754,22 @@ extern int lockdep_rtnl_is_held(void);
  * @p: The pointer to read, prior to dereferencing
  *
  * Do an rcu_dereference(p), but check caller either holds rcu_read_lock()
- * or RTNL
+ * or RTNL. Note : Please prefer rtnl_dereference() or rcu_dereference()
  */
 #define rcu_dereference_rtnl(p)					\
 	rcu_dereference_check(p, rcu_read_lock_held() ||	\
 				 lockdep_rtnl_is_held())
 
 /**
- * rtnl_dereference - rcu_dereference with debug checking
+ * rtnl_dereference - fetch RCU pointer when updates are prevented by RTNL
  * @p: The pointer to read, prior to dereferencing
  *
- * Do an rcu_dereference(p), but check caller holds RTNL
+ * Return the value of the specified RCU-protected pointer, but omit
+ * both the smp_read_barrier_depends() and the ACCESS_ONCE(), because
+ * caller holds RTNL.
  */
 #define rtnl_dereference(p)					\
-	rcu_dereference_check(p, lockdep_rtnl_is_held())
+	rcu_dereference_protected(p, lockdep_rtnl_is_held())
 
 extern void rtnetlink_init(void);
 extern void __rtnl_unlock(void);


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