linux-kernel - Re: [PATCH v2] printk: Add console owner and waiter logic to load balance console writes

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Message-ID: <20171102131059.4d8935a9@gandalf.local.home>
Date:   Thu, 2 Nov 2017 13:10:59 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "yuwang.yuwang" <yuwang.yuwang@...baba-inc.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v2] printk: Add console owner and waiter logic to load
 balance console writes

On Thu, 2 Nov 2017 13:06:05 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> +		raw_spin_lock(&console_owner_lock);
> +		waiter = console_waiter;
> +		console_owner = NULL;
> +		raw_spin_unlock(&console_owner_lock);
> +
> +		/*
> +		 * If there is a waiter waiting for us, then pass the
> +		 * rest of the work load over to that waiter.
> +		 */
> +		if (waiter)
> +			break;
> +

Hmm, do I need a READ_ONCE() here?

Can gcc do the load of console_waiter outside the spin lock where 
if (waiter) is done?

Although it doesn't really matter, but it just makes the code more
fragile if it can. Should this be:

		raw_spin_lock(&console_owner_lock);
		waiter = READ_ONCE(console_waiter);
		console_owner = NULL;
		raw_spin_unlock(&console_owner_lock);

		/*
		 * If there is a waiter waiting for us, then pass the
		 * rest of the work load over to that waiter.
		 */
		if (waiter)
			break;

 ?

-- Steve