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

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Date:	Sat, 16 Nov 2013 12:32:16 +0800
From:	Ding Tianhong <dthxman@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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

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.


> 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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