linux-kernel - Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

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Date:	Thu, 24 Apr 2008 23:30:00 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Mark Lord <lkml@....ca>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, npiggin@...e.de,
	torvalds@...ux-foundation.org, pavel@....cz
Subject: Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

On Thursday, 24 of April 2008, Mark Lord wrote:
> Jens Axboe wrote:
> > On Wed, Apr 23 2008, Mark Lord wrote:
> >> Jens Axboe wrote:
> >>> On Wed, Apr 23 2008, Mark Lord wrote:
> >>> ..
> >>>> The second bug, is that for the halt case at least,
> >>>> nobody waits for the other CPU to actually halt
> >>>> before continuing.. so we sometimes enter the shutdown
> >>>> code while other CPUs are still active.
> >>>>
> >>>> This causes some machines to hang at shutdown,
> >>>> unless CPU_HOTPLUG is configured and takes them offline
> >>>> before we get here.
> >>> I'm guessing there's a reason it doesn't pass '1' as the last argument,
> >>> because that would fix that issue?
> >> ..
> >>
> >> Undoubtedly -- perhaps the called CPU halts, and therefore cannot reply.  :)
> > 
> > Uhm yes, I guess stop_this_cpu() does exactly what the name implies :-)
> > 
> >> But some kind of pre-halt ack, perhaps plus a short delay by the caller
> >> after receipt of the ack, would probably suffice to kill that bug.
> >>
> >> But I really haven't studied this code enough to know,
> >> other than that it historically has been a sticky area
> >> to poke around in.
> > 
> > Something like this will close the window to right up until the point
> > where the other CPUs have 'almost' called halt().
> > 
> > diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> > index 5398385..94ec9bf 100644
> > --- a/arch/x86/kernel/smp.c
> > +++ b/arch/x86/kernel/smp.c
> > @@ -155,8 +155,9 @@ static void stop_this_cpu(void *dummy)
> >  	/*
> >  	 * Remove this CPU:
> >  	 */
> > -	cpu_clear(smp_processor_id(), cpu_online_map);
> >  	disable_local_APIC();
> > +	cpu_clear(smp_processor_id(), cpu_online_map);
> > +	smp_wmb();
> >  	if (hlt_works(smp_processor_id()))
> >  		for (;;) halt();
> >  	for (;;);
> > @@ -175,6 +176,12 @@ static void native_smp_send_stop(void)
> >  
> >  	local_irq_save(flags);
> >  	smp_call_function(stop_this_cpu, NULL, 0, 0);
> > +
> > +	while (cpus_weight(cpu_online_map) > 1) {
> > +		cpu_relax();
> > +		smp_rmb();
> > +	}
> > +
> >  	disable_local_APIC();
> >  	local_irq_restore(flags);
> >  }
> ..
> 
> Yup, that looks like it oughta work consistently.
> Now we just need to hear from some of the folks who
> have danced around this code in the past.
> 
> (added Pavel & Rafael to Cc:).

Well, it looks sane to me, but I'm not really an expert here.

Thanks,
Rafael
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