[tip/core/rcu,11/14] bonding/bond_main: Apply ACCESS_ONCE() to avoid sparse false positive

From: "Paul E. McKenney" <paulmck@linux.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.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: bridge@lists.linux-foundation.org
Cc: netdev@vger.kernel.org
---
 drivers/net/bonding/bond_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

于 2013/11/16 8:40, Paul E. McKenney 写道:
> From: "Paul E. McKenney" <paulmck@linux.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@intel.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Stephen Hemminger <stephen@networkplumber.org>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: bridge@lists.linux-foundation.org
> Cc: netdev@vger.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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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@linux.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@intel.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Stephen Hemminger <stephen@networkplumber.org>
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: bridge@lists.linux-foundation.org
> > Cc: netdev@vger.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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