linux-kernel - Re: [PATCH] rcu/exp: Fix check for idle context in rcu_exp

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Date:   Mon, 13 Dec 2021 11:20:12 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Neeraj Upadhyay <quic_neeraju@...cinc.com>, josh@...htriplett.org,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, joel@...lfernandes.org,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        urezki@...il.com, boqun.feng@...il.com
Subject: Re: [PATCH] rcu/exp: Fix check for idle context in rcu_exp_handler

On Mon, Dec 13, 2021 at 12:34:55PM +0100, Frederic Weisbecker wrote:
> On Mon, Dec 13, 2021 at 11:40:24AM +0530, Neeraj Upadhyay wrote:
> > For PREEMPT_RCU, the rcu_exp_handler() function checks
> > whether the current CPU is in idle, by calling
> > rcu_dynticks_curr_cpu_in_eqs(). However, rcu_exp_handler()
> > is called in IPI handler context. So, it should be checking
> > the idle context using rcu_is_cpu_rrupt_from_idle(). Fix this
> > by using rcu_is_cpu_rrupt_from_idle() instead of
> > rcu_dynticks_curr_cpu_in_eqs(). Non-preempt configuration
> > already uses the correct check.
> > 
> > Signed-off-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> 
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Queued for further review and testing, thank you both!

							Thanx, Paul

> > ---
> >  kernel/rcu/tree_exp.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index 237a79989aba..1568c8ef185b 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -656,7 +656,7 @@ static void rcu_exp_handler(void *unused)
> >  	 */
> >  	if (!depth) {
> >  		if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
> > -		    rcu_dynticks_curr_cpu_in_eqs()) {
> > +		    rcu_is_cpu_rrupt_from_idle()) {
> >  			rcu_report_exp_rdp(rdp);
> >  		} else {
> >  			WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
> > -- 
> > 2.17.1
> >