From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1753877AbcKIS5m (ORCPT <rfc822;w@1wt.eu>);
        Wed, 9 Nov 2016 13:57:42 -0500
Received: from foss.arm.com ([217.140.101.70]:33664 "EHLO foss.arm.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1751685AbcKIS5l (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 9 Nov 2016 13:57:41 -0500
Date: Wed, 9 Nov 2016 18:57:43 +0000
From: Will Deacon <will.deacon@arm.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Chris Metcalf <cmetcalf@mellanox.com>,
        Gilad Ben Yossef <giladb@mellanox.com>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Tejun Heo <tj@kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, Christoph Lameter <cl@linux.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Francis Giraldeau <francis.giraldeau@gmail.com>,
        Andi Kleen <andi@firstfloor.org>, Arnd Bergmann <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: task isolation discussion at Linux Plumbers
Message-ID: <20161109185743.GN17771@arm.com>
References: <1471382376-5443-1-git-send-email-cmetcalf@mellanox.com>
 <1605b087-2b3b-77c1-01ac-084e378f5f28@mellanox.com>
 <20161109014045.GI4127@linux.vnet.ibm.com>
 <CALCETrWjMrdJPHSjA64rJpVCaEtZCr-ZeE08sKP1-rbvw-4+tQ@mail.gmail.com>
 <20161109173808.GJ4127@linux.vnet.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161109173808.GJ4127@linux.vnet.ibm.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Hi Paul,

Just a couple of comments, but they be more suited to Andy.

On Wed, Nov 09, 2016 at 09:38:08AM -0800, Paul E. McKenney wrote:
> @@ -355,10 +373,33 @@ static bool rcu_dynticks_in_eqs_since(struct rcu_dynticks *rdtp, int snap)
>  static void rcu_dynticks_momentary_idle(void)
>  {
>  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> -	int special = atomic_add_return(2, &rdtp->dynticks);
> +	int special = atomic_add_return(2 * RCU_DYNTICK_CTRL_CTR,
> +					&rdtp->dynticks);
>  
>  	/* It is illegal to call this from idle state. */
> -	WARN_ON_ONCE(!(special & 0x1));
> +	WARN_ON_ONCE(!(special & RCU_DYNTICK_CTRL_CTR));
> +}
> +
> +/*
> + * Set the special (bottom) bit of the specified CPU so that it
> + * will take special action (such as flushing its TLB) on the
> + * next exit from an extended quiescent state.  Returns true if
> + * the bit was successfully set, or false if the CPU was not in
> + * an extended quiescent state.
> + */

Given that TLB maintenance on arm is handled in hardware (no need for IPI),
I'd like to avoid this work if at all possible. However, without seeing the
call site I can't tell if it's optional.

> +bool rcu_eqs_special_set(int cpu)
> +{
> +	int old;
> +	int new;
> +	struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> +
> +	do {
> +		old = atomic_read(&rdtp->dynticks);
> +		if (old & RCU_DYNTICK_CTRL_CTR)
> +			return false;
> +		new = old | RCU_DYNTICK_CTRL_MASK;
> +	} while (atomic_cmpxchg(&rdtp->dynticks, old, new) != old);
> +	return true;
>  }

Can this be a cmpxchg_relaxed? What is it attempting to order?

Will