linux-kernel - Re: [PATCH 1/3] stop_machine: kill __stop

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Message-ID: <1308248897.13240.272.camel@twins>
Date:	Thu, 16 Jun 2011 20:28:17 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Tejun Heo <tj@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] stop_machine: kill __stop_machine()

On Thu, 2011-06-16 at 11:17 -0700, Suresh Siddha wrote:

> > > Peter, So it looks like we are allowing a new cpu to come online in
> > > parallel, while we poke the text? Isn't it a problem? What am I missing?
> > 
> > the caller already did get_online_cpus(), 
> > 
> > do_optimize_kprobes()
> >   get_online_cpus()
> >   arch_optimize_kprobes()
> >     text_poke_smp_batch()
> >   put_online_cpus()
> 
> So the circular dependency reported is not possible in practice right?

why not? get_online_cpus() takes a mutex, that connects smp_alt and
text_mutex and can cause a deadlock.
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