linux-kernel - Re: [PATCH tip/core/rcu 11/14] bonding/bond_main: Apply ACCESS

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Message-ID: <20131116152119.GB4138@linux.vnet.ibm.com>
Date:	Sat, 16 Nov 2013 07:21:19 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ding Tianhong <dthxman@...il.com>
Cc:	linux-kernel@...r.kernel.org, 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, Stephen Hemminger <stephen@...workplumber.org>,
	"David S. Miller" <davem@...emloft.net>,
	bridge@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH tip/core/rcu 11/14] bonding/bond_main: Apply
 ACCESS_ONCE() to avoid sparse false positive

On Sat, Nov 16, 2013 at 12:32:16PM +0800, Ding Tianhong wrote:
> 于 2013/11/16 8:40, Paul E. McKenney 写道:
> > 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() 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.
> 
> I think it is fit for net-next.

Thank you!

If this is queued there, I would be happy to drop it from my tree.
There are no dependencies on anything in my tree.

							Thanx, Paul

> > 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 | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> > index 72df399c4ab3..bbd7fd3e46fe 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) {
> > @@ -1801,7 +1802,7 @@ 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
> 

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