On Tue, 18 Oct 2016 19:30:46 +0200
Peter Zijlstra <pet...@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();
>       }
>  

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