On Tue, 18 Oct 2016 19:08:33 +0200
Peter Zijlstra <pet...@infradead.org> 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) <pet...@infradead.org>
> ---

hehe, I have the exact same patch for my -rt work.

-- Steve

>  kernel/printk/printk.c |   16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> --- 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;
> +
>  static int early_vprintk(const char *fmt, va_list args)
>  {
> +     int n, cpu, old;
>       char buf[512];
> -     int n;
> +
> +     cpu = get_cpu();
> +     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);
> +     put_cpu();
> +
>       return n;
>  }
>  
> 

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