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    Subject[PATCH 4.12 58/84] rcu: Add memory barriers for NOCB leader wakeup
    Date
    4.12-stable review patch.  If anyone has any objections, please let me know.

    ------------------

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

    commit 6b5fc3a1331810db407c9e0e673dc1837afdc9d0 upstream.

    Wait/wakeup operations do not guarantee ordering on their own. Instead,
    either locking or memory barriers are required. This commit therefore
    adds memory barriers to wake_nocb_leader() and nocb_leader_wait().

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Tested-by: Krister Johansen <kjlx@templeofstupid.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/rcu/tree_plugin.h | 2 ++
    1 file changed, 2 insertions(+)

    --- a/kernel/rcu/tree_plugin.h
    +++ b/kernel/rcu/tree_plugin.h
    @@ -1769,6 +1769,7 @@ static void wake_nocb_leader(struct rcu_
    if (READ_ONCE(rdp_leader->nocb_leader_sleep) || force) {
    /* Prior smp_mb__after_atomic() orders against prior enqueue. */
    WRITE_ONCE(rdp_leader->nocb_leader_sleep, false);
    + smp_mb(); /* ->nocb_leader_sleep before swake_up(). */
    swake_up(&rdp_leader->nocb_wq);
    }
    }
    @@ -2023,6 +2024,7 @@ wait_again:
    * nocb_gp_head, where they await a grace period.
    */
    gotcbs = false;
    + smp_mb(); /* wakeup before ->nocb_head reads. */
    for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_follower) {
    rdp->nocb_gp_head = READ_ONCE(rdp->nocb_head);
    if (!rdp->nocb_gp_head)

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