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

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Message-ID: <20110616182805.GD2611@htj.dyndns.org>
Date:	Thu, 16 Jun 2011 20:28:05 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"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()

Hello,

On Thu, Jun 16, 2011 at 11:17:40AM -0700, Suresh Siddha wrote:
> On Thu, 2011-06-16 at 10:55 -0700, Peter Zijlstra wrote:
> > the caller already did get_online_cpus(), 
> > 
> > do_optimize_kprobes()
> >   get_online_cpus()
> >   arch_optimize_kprobes()
> >     text_poke_smp_batch()
> >   put_online_cpus()

Peter, I don't think it's that simple.  get_online_cpus() itself can't
create circular dependency by itself.  It allows recursing.  The chain
involves cpu_hotplug_begin() which returns with hotplug mutex held.

> So the circular dependency reported is not possible in practice right?
> 
> Above patch is working around a false positive.

But I don't think the lockdep warning is spurious.  text_mutex ->
hotplug.lock dependency is created during kprobe patching.
hotplug.lock -> smp_alt during CPU hotplug, and then the alternative
code calling get_online_cpus() creates reverse dependency through
get_online_cpus().  So, the reasoning and comment are wrong but
grapping hotplug mutex there does create a circular dependency.

This probably can be resolved in prettier way but let's leave it alone
for now.

Thanks.

-- 
tejun
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