Re: [PATCH] rcu/tree: Fix self wakeups for grace period kthread

[Neeraj Upadhyay]

On 3/12/19 7:20 AM, Steven Rostedt wrote:
> On Fri,  8 Mar 2019 15:16:18 +0530
> Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> wrote:
>
> > Update the code to match the comment that self wakeup of
> > grace period kthread is allowed from interrupt handler, and
> > softirq handler, running in the grace period kthread's
> > context. Present code allows self wakeups from all
> > interrupt contexts - nmi, softirq and hardirq contexts.
>
> That's not actually the issue. But it appears that we return if we
> simply have BH disabled, which I don't think we want, and we don't care
> about NMI as NMI should never call this code.
>
> I think your patch is correct, but the change log is not.
>
> -- Steve

Hi Steve, sorry, I don't understand fully, why we want to not return in 
BH disabled case. From the commit logs and lkml discussion, there is a 
case where GP kthread is interrupted in the wait event path and 
rcu_gp_kthread_wake() is called in softirq handler (I am not sure about 
interrupt handler case; how rcu_gp_kthread_wake() is called from that path).

https://github.com/torvalds/linux/commit/1d1f898df6586c5ea9aeaf349f13089c6fa37903

Thanks
Neeraj
> > Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> > ---
> >   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 acd6ccf..57cac6d 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1585,7 +1585,7 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
> >   static void rcu_gp_kthread_wake(void)
> >   {
> >   	if ((current == rcu_state.gp_kthread &&
> > -	     !in_interrupt() && !in_serving_softirq()) ||
> > +	     !in_irq() && !in_serving_softirq()) ||
> >   	    !READ_ONCE(rcu_state.gp_flags) ||
> >   	    !rcu_state.gp_kthread)
> >   		return;

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