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Date: Tue, 18 Oct 2016 13:53:29 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Petr Mladek <pmladek@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        Tejun Heo <tj@kernel.org>, Calvin Owens <calvinowens@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] early_printk: Add simple serialization to
 early_vprintk()
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On Tue, 18 Oct 2016 19:30:46 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Oct 18, 2016 at 01:19:51PM -0400, Steven Rostedt wrote:

> Looking at this again, we really should not spin using cmpxchg(), that
> thrashes the cacheline.
> 
> The below is slightly better spinning... then again, this isn't
> performance code.

Yeah, this is similar to the hack I had.

May want to fold this in your previous version.

-- Steve

> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -376,10 +376,16 @@ static int early_vprintk(const char *fmt
>  
>  	cpu = get_cpu();
>  	for (;;) {
> -		old = cmpxchg(&early_printk_cpu, -1, cpu);
> -		if (old == -1 || old == cpu)
> +		old = READ_ONCE(early_printk_cpu);
> +		if (old == cpu)
>  			break;
>  
> +		if (old == -1) {
> +			old = cmpxchg(&early_printk_cpu, -1, cpu);
> +			if (old == -1 || old == cpu)
> +				break;
> +		}
> +
>  		cpu_relax();
>  	}
>