linux-kernel - Re: [PATCH 3/3] early_printk: Add simple serialization to early

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Message-ID: <20161129141054.GD21230@pathway.suse.cz>
Date:   Tue, 29 Nov 2016 15:10:55 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] early_printk: Add simple serialization to
 early_vprintk()

On Tue 2016-10-18 19:08:33, Peter Zijlstra wrote:
> In order to avoid multiple CPUs banging on the serial port at the same
> time, add simple serialization. This explicitly deals with nested
> contexts (like IRQs etc.).
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Makes sense. Just a small comment below.

Reviewd-by: Petr Mladek <pmladek@...e.com>

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -356,14 +356,28 @@ static int __init force_early_printk_set
>  }
>  early_param("force_early_printk", force_early_printk_setup);
>  
> +static int early_printk_cpu = -1;

[...]

> +	for (;;) {
> +		old = cmpxchg(&early_printk_cpu, -1, cpu);
> +		if (old == -1 || old == cpu)
> +			break;
> +
> +		cpu_relax();
> +	}
>  
>  	n = vscnprintf(buf, sizeof(buf), fmt, args);
>  	early_console->write(early_console, buf, n);
>  
> +	smp_store_release(&early_printk_cpu, old);

checkpatch.pl complains about using a barrier without a comment.
The code is simple but it still might help to add something like:

	/* Releasing early_printk_cpu custom lock. */

Best Regards,
Petr