Re: [PATCH 3/3] early_printk: Add simple serialization to early_vprintk()

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 28 Sep 2017 14:18:26 +0200
Peter Zijlstra <peterz@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) <peterz@infradead.org>
> ---
>  kernel/printk/printk.c |   35 ++++++++++++++++++++++++++++++++++-
>  1 file changed, 34 insertions(+), 1 deletion(-)
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -378,14 +378,47 @@ 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();
> +	/*
> +	 * Test-and-Set inter-cpu spinlock with recursion.
> +	 */
> +	for (;;) {
> +		/*
> +		 * c-cas to avoid the exclusive bouncing on spin.
> +		 * Depends on the memory barrier implied by cmpxchg
> +		 * for ACQUIRE semantics.
> +		 */
> +		old = READ_ONCE(early_printk_cpu);
> +		if (old == -1) {

If old != -1 and old != cpu, is it possible that the CPU could have
fetched an old value, and never try to fetch it again?

The cmpxchg memory barrier only happens when old == -1.

-- Steve

> +			old = cmpxchg(&early_printk_cpu, -1, cpu);
> +			if (old == -1)
> +				break;
> +		}
> +		/*
> +		 * Allow recursion for interrupts and the like.
> +		 */
> +		if (old == cpu)
> +			break;
> +
> +		cpu_relax();
> +	}
>  
>  	n = vscnprintf(buf, sizeof(buf), fmt, args);
>  	early_console->write(early_console, buf, n);
>  
> +	/*
> +	 * Unlock -- in case @old == @cpu, this is a no-op.
> +	 */
> +	smp_store_release(&early_printk_cpu, old);
> +	put_cpu();
> +
>  	return n;
>  }
>  
>