linux-kernel - Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates

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Date:   Wed, 28 Jul 2021 14:23:05 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     paulmck <paulmck@...nel.org>
Cc:     rcu <rcu@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>, Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and
 updates

----- On Jul 28, 2021, at 1:37 PM, paulmck paulmck@...nel.org wrote:
[...]
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 42a0032dd99f7..c87b3a271d65b 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -251,6 +251,15 @@ void rcu_softirq_qs(void)
> 	rcu_tasks_qs(current, false);
> }
> 
> +/*
> + * Increment the current CPU's rcu_data structure's ->dynticks field
> + * with ordering.  Return the new value.
> + */
> +static noinstr unsigned long rcu_dynticks_inc(int incby)
> +{
> +	return arch_atomic_add_return(incby, this_cpu_ptr(&rcu_data.dynticks));
> +}
> +

[...]

> @@ -308,7 +317,7 @@ static void rcu_dynticks_eqs_online(void)
> 
> 	if (atomic_read(&rdp->dynticks) & 0x1)
> 		return;

Can the thread be migrated at this point ? If yes, then
the check and the increment may happen on different cpu's rdps. Is
that OK ?

> -	atomic_inc(&rdp->dynticks);
> +	rcu_dynticks_inc(1);
> }

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com