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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 18, 2016 at 01:19:51PM -0400, Steven Rostedt wrote:
> 
> hehe, I have the exact same patch for my -rt work.

> > +static int early_printk_cpu = -1;
> > +
> >  static int early_vprintk(const char *fmt, va_list args)
> >  {
> > +	int n, cpu, old;
> >  	char buf[512];
> > +
> > +	cpu = get_cpu();
> > +	for (;;) {
> > +		old = cmpxchg(&early_printk_cpu, -1, cpu);
> > +		if (old == -1 || old == cpu)
> > +			break;

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.

--- 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();
 	}