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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 03, 2017 at 06:24:22PM -0400, Steven Rostedt wrote:
> On Thu, 28 Sep 2017 14:18:26 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> >  static int early_vprintk(const char *fmt, va_list args)
> >  {
> > +	int n, cpu, old;
> >  	char buf[512];
> > +
> > +	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?

What? If old != -1 and old != cpu, we'll hit the cpu_relax() and do the
READ_ONCE() again. The READ_ONCE() guarantees we'll do the load again,
as does the barrier() implied by cpu_relax().

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

Yeah, so?

> > +			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;
> >  }