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    Subject[PATCH tip/core/rcu 12/14] bonding/bond_alb.c: Apply ACCESS_ONCE() to avoid sparse false positive
    Date
    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 use in
    bond_alb_handle_active_change() is legitimate: It is assigning a pointer
    to an element from an RCU-protected list, and all elements of this list
    are already visible to caller.

    This commit therefore silences this false positive by laundering the
    pointer using ACCESS_ONCE() as suggested by Eric Dumazet and Josh
    Triplett.

    Reported-by: kbuild test robot <fengguang.wu@intel.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: "David S. Miller" <davem@davemloft.net>
    Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
    Cc: James Morris <jmorris@namei.org>
    Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
    Cc: Patrick McHardy <kaber@trash.net>
    Cc: netdev@vger.kernel.org
    ---
    drivers/net/bonding/bond_alb.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/drivers/net/bonding/bond_alb.c b/drivers/net/bonding/bond_alb.c
    index 91f179d5135c..67d3b2893aa3 100644
    --- a/drivers/net/bonding/bond_alb.c
    +++ b/drivers/net/bonding/bond_alb.c
    @@ -1667,7 +1667,8 @@ void bond_alb_handle_active_change(struct bonding *bond, struct slave *new_slave
    }

    swap_slave = bond->curr_active_slave;
    - 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;

    if (!new_slave || list_empty(&bond->slave_list))
    return;
    --
    1.8.1.5


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