Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value

Paul E. McKenney
2014-07-23T12:26:15Z

On Wed, Jul 23, 2014 at 01:09:48AM -0400, Pranith Kumar wrote:
> When the gp_kthread wakes up from the wait event, it returns 0 if the wake up is
> due to the condition having been met. This commit checks this return value
> for a spurious wake up before calling rcu_gp_init().
>
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>

How does this added check help? I don't see that it does. If the flag
is set, we want to wake up. If we get a spurious wakeup, but then the
flag gets set before we actually wake up, we still want to wake up.

So sorry, but no.

Thanx, Paul

PS. BTW, the ones that I am not immediately responding to are those
that I will recheck later on and possibly accept.

> ---
> kernel/rcu/tree.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f8847d9..3b20ad2 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1790,11 +1790,11 @@ static int __noreturn rcu_gp_kthread(void *arg)
> ACCESS_ONCE(rsp->gpnum),
> TPS("reqwait"));
> rsp->gp_state = RCU_GP_WAIT_GPS;
> - wait_event_interruptible(rsp->gp_wq,
> + ret = wait_event_interruptible(rsp->gp_wq,
> ACCESS_ONCE(rsp->gp_flags) &
> RCU_GP_FLAG_INIT);
> /* Locking provides needed memory barrier. */
> - if (rcu_gp_init(rsp))
> + if (!ret && rcu_gp_init(rsp))
> break;
> cond_resched();
> flush_signals(current);
> --
> 2.0.0.rc2
>