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

[Peter Zijlstra]

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();
        }