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

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Date:   Tue, 12 Mar 2019 07:09:23 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/tree: Fix self wakeups for grace period kthread

On Tue, Mar 12, 2019 at 05:25:28PM +0530, Neeraj Upadhyay wrote:
> On 3/12/19 7:20 AM, Steven Rostedt wrote:
> >On Fri,  8 Mar 2019 15:16:18 +0530
> >Neeraj Upadhyay <neeraju@...eaurora.org> 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.

How about this?

	The current rcu_gp_kthread_wake() function uses in_interrupt()
	and thus does a self-wakeup from all interrupt contexts,
	including the pointless case where the GP kthread happens to be
	running with bottom halves disabled, along with the impossible
	case where the GP kthread is running within an NMI handler (you
	are not supposed to invoke rcu_gp_kthread_wake() from within an
	NMI handler.  This commit therefore replaces the in_interrupt()
	with in_irq(), so that the self-wakeups happen only from handlers
	for hardware interrupts and softirqs.  This also makes the code
	match the comment.

							Thanx, Paul

> >-- 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@...eaurora.org>
> >>---
> >>  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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>