linux-kernel - Re: + pm-introduce-new-interfaces-schedule_work_on-and-queue_work

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Message-ID: <20080805195706.GD7939@elf.ucw.cz>
Date:	Tue, 5 Aug 2008 21:57:06 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Zhang Rui <rui.zhang@...el.com>, Gautham R Shenoy <ego@...ibm.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	harbour@...nx.od.ua, rjw@...k.pl
Subject: Re: +
	pm-introduce-new-interfaces-schedule_work_on-and-queue_work_on.patch
	added to -mm tree

Hi!

> > > This means that
> > >
> > >         pm-schedule-sysrq-poweroff-on-boot-cpu.patch
> > >
> > > is not 100% right. It is still possible to hang/deadlock if we race
> > > with cpu_down(first_cpu(cpu_online_map)).
> >
> > Yes, you're right.
> > But then should we fix disable_nonboot_cpus as well?
> >
> > int disable_nonboot_cpus(void)
> > {
> >         first_cpu = first_cpu(cpu_online_map);
> > 	...
> >
> >         for_each_online_cpu(cpu) {
> >                 if (cpu == first_cpu)
> >                         continue;
> >                 error = _cpu_down(cpu, 1);
> > 		...
> >         }
> > 	...
> > }
> 
> Note that disable_nonboot_cpus() does first_cpu = first_cpu() under
> cpu_maps_update_begin(), so we can't race with cpu-hotplug.
> 
> However, this afaics means that its name is wrong, and
> printk("Disabling non-boot CPUs ...\n") is not right too.
> What it does is disable_all_but_one_cpus().

I thought that first cpu is defined to be boot cpu?

> And, it is not clear why disable_nonboot_cpus() assumes that
> all but first_cpu(cpu_online_map) must have .hotpluggable == 1.

Where does it assume that?

It will fail if some CPUs can't be unplugged, and I'm afraid suspend
can't work in such case... 

> And, if one of the callers really need to preserve the boot CPU,
> I don't understand how it is guaranteed it must be first_cpu().

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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