linux-kernel - Re: [PATCH 2/2] rcu: Remove needless preemption disablement in rcu_all

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Date:   Tue, 6 Jul 2021 09:51:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 2/2] rcu: Remove needless preemption disablement in
 rcu_all_qs()

On Tue, Jul 06, 2021 at 01:43:44AM +0200, Frederic Weisbecker wrote:
> The preemption is already disabled when we write rcu_data.rcu_urgent_qs.
> We can use __this_cpu_write() directly, although that path is mostly
> used when CONFIG_PREEMPT=n.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> Cc: Uladzislau Rezki <urezki@...il.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> ---
>  kernel/rcu/tree_plugin.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 27b74352cccf..38b3d01424d7 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -871,7 +871,7 @@ void rcu_all_qs(void)
>  		preempt_enable();
>  		return;
>  	}
> -	this_cpu_write(rcu_data.rcu_urgent_qs, false);
> +	__this_cpu_write(rcu_data.rcu_urgent_qs, false);

There's another subtle difference between this_cpu_write() and
__this_cpu_write() aside from preempt. this_cpu_write() is also
IRQ-safe, while __this_cpu_write() is not.

I've not looked at the usage here to see if that is relevant, but the
Changelog only mentioned the preempt side of things, and that argument
is incomplete in general.