linux-kernel - Re: [PATCH v3 1/5] x86: initialize secondary CPU only if master CPU will wait for it

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Date:	Mon, 14 Apr 2014 12:21:39 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, bp@...e.de,
	paul.gortmaker@...driver.com, JBeulich@...e.com, prarit@...hat.com,
	drjones@...hat.com, toshi.kani@...com, riel@...hat.com,
	gong.chen@...ux.intel.com, andi@...stfloor.org, lenb@...nel.org,
	rjw@...ysocki.net, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v3 1/5] x86: initialize secondary CPU only if master CPU
 will wait for it

On Mon, 14 Apr 2014 12:03:35 +0200
Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Igor Mammedov <imammedo@...hat.com> wrote:
> 
> > On Mon, 14 Apr 2014 11:16:00 +0200
> > Ingo Molnar <mingo@...nel.org> wrote:
> > 
> > > 
> > > * Igor Mammedov <imammedo@...hat.com> wrote:
> > > 
> > > >  	/*
> > > > +	 * wait for ACK from master CPU before continuing
> > > > +	 * with AP initialization
> > > > +	 */
> > > > +	cpumask_set_cpu(cpu, cpu_initialized_mask);
> > > > +	while (!cpumask_test_cpu(cpu, cpu_callout_mask))
> > > > +		cpu_relax();
> > > 
> > > > +	/*
> > > > +	 * wait for ACK from master CPU before continuing
> > > > +	 * with AP initialization
> > > > +	 */
> > > > +	cpumask_set_cpu(cpu, cpu_initialized_mask);
> > > > +	while (!cpumask_test_cpu(cpu, cpu_callout_mask))
> > > > +		cpu_relax();
> > > 
> > > That repetitive pattern could be stuck into a properly named helper 
> > > inline function.
> > sure
> > 
> > > (Also, before the cpumask_set_cpu() we should probably do a WARN_ON() 
> > > if the bit is already set.)
> > The reason why there is no any WARN_ON or likes is that printk is quite
> > complicated, takes looks and so on. [...]
> 
> [ Yeah, I too heard that printk(), like a pretty girl, is complicated
>   and makes people look twice. ]
> 
> > [...] So it's not safe at this point since
> > CPU could be shot down by any time by INIT/SIPI until it's out of 
> > cpu_callout_mask loop.
> 
> Not sure where you got that from, but it's not a valid concern really: 
> the only place where we don't want to do a printk() is in printk code 
> itself.
> 
> Debug warnings, by definition, should never trigger. If they trigger 
> then they will very likely not cause lockups, but will cause the bug 
> to be fixed.
ok, I'll add WARN_ON in cpu_init() as you've suggested.

> Thanks,
> 
> 	Ingo


-- 
Regards,
  Igor
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