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

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Message-ID: <20140414170346.35ad01cb@nial.usersys.redhat.com>
Date:	Mon, 14 Apr 2014 17:03:46 +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 16:51:19 +0200
Ingo Molnar <mingo@...nel.org> wrote:

> 
> * 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 for your patience
I'll repost fixed and tested series in a minute

> 
> Thanks,
> 
> 	Ingo

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