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Date:   Wed, 28 Jul 2021 14:23:05 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     paulmck <paulmck@kernel.org>
Cc:     rcu <rcu@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@fb.com>, Ingo Molnar <mingo@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>,
        Eric Dumazet <edumazet@google.com>,
        fweisbec <fweisbec@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
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In-Reply-To: <20210728173715.GA9416@paulmck-ThinkPad-P17-Gen-1>
References: <20210721202042.GA1472052@paulmck-ThinkPad-P17-Gen-1> <20210721202127.2129660-4-paulmck@kernel.org> <20210728173715.GA9416@paulmck-ThinkPad-P17-Gen-1>
Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and
 updates
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Thread-Topic: Weaken ->dynticks accesses and updates
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----- On Jul 28, 2021, at 1:37 PM, paulmck paulmck@kernel.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