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    SubjectRe: [PATCH 10/16] rcu: Check for RCU_FLAG_GP_INIT bit in gp_flags for spurious wakeup
    On Wed, Jul 23, 2014 at 01:09:47AM -0400, Pranith Kumar wrote:
    > rcu_gp_init() is called once the gp_kthread wakes up on the condition that
    > the init bit is set in the gp_flags. For a spurious wakeup we need to check that
    > it is actually set. This commit ensures that the RCU_GP_FLAG_INIT bit is set in
    > gp_flags.
    >
    > Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>

    The trick here is that the other bits have no meaning if we are not yet
    in a grace period. If any get spuriously set, for example, by a race
    between force_quiescent_state() and the end of a grace period, we really
    do want to clear them all.

    So sorry, but no.

    Thanx, Paul

    > ---
    > kernel/rcu/tree.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    > index 8d598a2..f8847d9 100644
    > --- a/kernel/rcu/tree.c
    > +++ b/kernel/rcu/tree.c
    > @@ -1600,7 +1600,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
    > rcu_bind_gp_kthread();
    > raw_spin_lock_irq(&rnp->lock);
    > smp_mb__after_unlock_lock();
    > - if (!ACCESS_ONCE(rsp->gp_flags)) {
    > + if (!(ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_INIT)) {
    > /* Spurious wakeup, tell caller to go back to sleep. */
    > raw_spin_unlock_irq(&rnp->lock);
    > return 0;
    > --
    > 2.0.0.rc2
    >



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