linux-kernel - Re: [RFC] rcu: Prevent expedite reporting within RCU read-side section

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Date:   Tue, 6 Mar 2018 21:43:19 +0900
From:   Byungchul Park <max.byungchul.park@...il.com>
To:     Byungchul Park <byungchul.park@....com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:     rostedt@...dmis.org, josh@...htriplett.org, jiangshanlai@...il.com,
        kernel-team@....com,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC] rcu: Prevent expedite reporting within RCU read-side section

On Mar 6, 2018 2:34 PM, "Byungchul Park" <byungchul.park@....com> wrote:
>
> Hello Paul and RCU folks,
>
> I am afraid I correctly understand and fix it. But I really wonder why
> sync_rcu_exp_handler() reports the quiescent state even in the case that
> current task is within a RCU read-side section. Do I miss something?

Hello,

I missed the fact that the original code is anyway safe because
the case is gonna be handled properly in rcu_read_unlock().

This patch just makes unnecessary spin lock/unlock within *report*()
avoided. Please ignore this if you don't think it's that worthy. I am
also not sure if it is.

Sorry bothering you. And thanks.

> If I correctly understand it and you agree with it, I can add more logic
> which make it more expedited by boosting current or making it urgent
> when we fail to report the quiescent state on the IPI.
>
> ----->8-----
> From 0b0191f506c19ce331a1fdb7c2c5a00fb23fbcf2 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul.park@....com>
> Date: Tue, 6 Mar 2018 13:54:41 +0900
> Subject: [RFC] rcu: Prevent expedite reporting within RCU read-side section
>
> We report the quiescent state for this cpu if it's out of RCU read-side
> section at the moment IPI was just fired during the expedite process.
>
> However, current code reports the quiescent state even in the case:
>
>    1) the current task is still within a RCU read-side section
>    2) the current task has been blocked within the RCU read-side section
>
> Since we don't get to the quiescent state yet in the case, we shouldn't
> report it but check it another time.
>
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/rcu/tree_exp.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 73e1d3d..cc69d14 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -731,13 +731,13 @@ static void sync_rcu_exp_handler(void *info)
>         /*
>          * We are either exiting an RCU read-side critical section (negative
>          * values of t->rcu_read_lock_nesting) or are not in one at all
> -        * (zero value of t->rcu_read_lock_nesting).  Or we are in an RCU
> -        * read-side critical section that blocked before this expedited
> -        * grace period started.  Either way, we can immediately report
> -        * the quiescent state.
> +        * (zero value of t->rcu_read_lock_nesting). We can immediately
> +        * report the quiescent state.
>          */
> -       rdp = this_cpu_ptr(rsp->rda);
> -       rcu_report_exp_rdp(rsp, rdp, true);
> +       if (t->rcu_read_lock_nesting <= 0) {
> +               rdp = this_cpu_ptr(rsp->rda);
> +               rcu_report_exp_rdp(rsp, rdp, true);
> +       }
>  }
>
>  /**
> --
> 1.9.1
>