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 16:51:19 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Igor Mammedov <imammedo@...hat.com>
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


* Igor Mammedov <imammedo@...hat.com> wrote:

> 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.)
>
> WARN_ON will never be triggered here since bit is always cleared by 
> master CPU before AP gets here. There is no harm keeping WARN_ON 
> though, do you still want it be here?

The previous code panic()ed on this condition - so it makes sense to 
at least keep a WARN_ON(). That it won't ever trigger is good:

> It could be useful to put WARN_ON in do_boot_cpu() before bit is 
> cleared, so that user would see that he tries to online AP which has 
> failed previous time. It's not really necessary since failed to 
> online attempt reported in logs at ERR level now, see patch 2/5.

WARN_ON()s are not used to communicate with users, they are used to 
show developers that there's a _bug_ in the code!

So a WARN_ON() not triggering, ever, is a good thing.

Thanks,

	Ingo
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