Re: [PATCH 10/16] rcu: Check for RCU_FLAG_GP_INIT bit in gp_flags for spurious wakeup

Paul E. McKenney
2014-07-23T12:23:44Z

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
>