linux-kernel - Re: [PATCH] rcu: Avoid to modify mask_ofl_ipi in sync_rcu_exp_select_node

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Message-ID: <20191009154446.GA2689@paulmck-ThinkPad-P72>
Date:   Wed, 9 Oct 2019 08:44:46 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        elver@...gle.com, Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, rcu@...r.kernel.org
Subject: Re: [PATCH] rcu: Avoid to modify mask_ofl_ipi in
 sync_rcu_exp_select_node_cpus()

On Tue, Oct 08, 2019 at 12:30:28PM -0400, Joel Fernandes wrote:
> On Tue, Oct 08, 2019 at 01:01:40PM +0800, Boqun Feng wrote:
> > "mask_ofl_ipi" is used for iterate CPUs which IPIs are needed to send
> > to, however in the IPI sending loop, "mask_ofl_ipi" along with another
> > variable "mask_ofl_test" might also get modified to record which CPU's
> > quiesent state can be reported by sync_rcu_exp_select_node_cpus(). Two
> > variables seems to be redundant for such a propose, so this patch clean
> > things a little by solely using "mask_ofl_test" for recording and
> > "mask_ofl_ipi" for iteration. This would improve the readibility of the
> > IPI sending loop in sync_rcu_exp_select_node_cpus().
> > 
> > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> > ---
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Applied, thank you!

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> >  kernel/rcu/tree_exp.h | 13 ++++++-------
> >  1 file changed, 6 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index 69c5aa64fcfd..212470018752 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -387,10 +387,10 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
> >  		}
> >  		ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
> >  		put_cpu();
> > -		if (!ret) {
> > -			mask_ofl_ipi &= ~mask;
> > +		/* The CPU responses the IPI, and will report QS itself */
> > +		if (!ret)
> >  			continue;
> > -		}
> > +
> >  		/* Failed, raced with CPU hotplug operation. */
> >  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
> >  		if ((rnp->qsmaskinitnext & mask) &&
> > @@ -401,13 +401,12 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
> >  			schedule_timeout_uninterruptible(1);
> >  			goto retry_ipi;
> >  		}
> > -		/* CPU really is offline, so we can ignore it. */
> > -		if (!(rnp->expmask & mask))
> > -			mask_ofl_ipi &= ~mask;
> > +		/* CPU really is offline, and we need its QS to pass GP. */
> > +		if (rnp->expmask & mask)
> > +			mask_ofl_test |= mask;
> >  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> >  	}
> >  	/* Report quiescent states for those that went offline. */
> > -	mask_ofl_test |= mask_ofl_ipi;
> >  	if (mask_ofl_test)
> >  		rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
> >  }
> > -- 
> > 2.23.0
> >