netdev - [PATCH v3 tip/core/rcu 12/14] bonding/bond_main: Apply ACCESS

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Message-Id: <1381533451-29018-12-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Fri, 11 Oct 2013 16:17:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	"David S. Miller" <davem@...emloft.net>,
	bridge@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: [PATCH v3 tip/core/rcu 12/14] bonding/bond_main: Apply ACCESS_ONCE() to avoid sparse false positive

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

The sparse checking for rcu_assign_pointer() was recently upgraded
to reject non-__kernel address spaces.  This also rejects __rcu,
which is almost always the right thing to do.  However, the uses in
bond_change_active_slave(), bond_enslave(), and __bond_release_one()
are legitimate: They are assigning a pointer to an element from an
RCU-protected list (or a NULL pointer), and all elements of this list
are already visible to caller.

This commit therefore silences these false positives either by laundering
the pointers using ACCESS_ONCE() as suggested by Eric Dumazet and Josh
Triplett, or by using RCU_INIT_POINTER() for NULL pointer assignments.

Reported-by: kbuild test robot <fengguang.wu@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Stephen Hemminger <stephen@...workplumber.org>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: bridge@...ts.linux-foundation.org
Cc: netdev@...r.kernel.org
---
 drivers/net/bonding/bond_main.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 72df399c4ab3..e4270ae1c0a8 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -890,7 +890,8 @@ void bond_change_active_slave(struct bonding *bond, struct slave *new_active)
 		if (new_active)
 			bond_set_slave_active_flags(new_active);
 	} else {
-		rcu_assign_pointer(bond->curr_active_slave, new_active);
+		/* Both --rcu and visible, so ACCESS_ONCE() is OK. */
+		ACCESS_ONCE(bond->curr_active_slave) = new_active;
 	}
 
 	if (bond->params.mode == BOND_MODE_ACTIVEBACKUP) {
@@ -1601,7 +1602,8 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev)
 		 * so we can change it without calling change_active_interface()
 		 */
 		if (!bond->curr_active_slave && new_slave->link == BOND_LINK_UP)
-			rcu_assign_pointer(bond->curr_active_slave, new_slave);
+			/* Both --rcu and visible, so ACCESS_ONCE() is OK. */
+			ACCESS_ONCE(bond->curr_active_slave) = new_slave;
 
 		break;
 	} /* switch(bond_mode) */
@@ -1801,7 +1803,7 @@ static int __bond_release_one(struct net_device *bond_dev,
 	}
 
 	if (all) {
-		rcu_assign_pointer(bond->curr_active_slave, NULL);
+		RCU_INIT_POINTER(bond->curr_active_slave, NULL);
 	} else if (oldcurrent == slave) {
 		/*
 		 * Note that we hold RTNL over this sequence, so there
-- 
1.8.1.5

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