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

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Date:	Thu, 16 Jun 2011 11:17:40 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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 10:55 -0700, Peter Zijlstra wrote:
> On Thu, 2011-06-16 at 10:37 -0700, Suresh Siddha wrote:
> > On Thu, 2011-06-16 at 05:12 -0700, Peter Zijlstra wrote:
> > > On Tue, 2011-06-14 at 19:06 +0200, Tejun Heo wrote:
> > > Please have a look at: 
> > > 
> > > ---
> > > commit d91309f69b7bdb64aeb30106fde8d18c5dd354b5
> > > Author: Peter Zijlstra <peterz@...radead.org>
> > > Date:   Fri Feb 11 22:07:46 2011 +0100
> > > 
> > >     x86: Fix text_poke_smp_batch() deadlock
> > >     
> > >     Fix this deadlock - we are already holding the mutex:
> > >     
> > ...
> > > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> > > index 1236085..7038b95 100644
> > > --- a/arch/x86/kernel/alternative.c
> > > +++ b/arch/x86/kernel/alternative.c
> > > @@ -671,7 +671,7 @@ void __kprobes text_poke_smp_batch(struct text_poke_param *params, int n)
> > >  
> > >  	atomic_set(&stop_machine_first, 1);
> > >  	wrote_text = 0;
> > > -	stop_machine(stop_machine_text_poke, (void *)&tpp, NULL);
> > > +	__stop_machine(stop_machine_text_poke, (void *)&tpp, NULL);
> > >  }
> > >  
> > >  #if defined(CONFIG_DYNAMIC_FTRACE) || defined(HAVE_JUMP_LABEL)
> > > 
> > 
> > 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?

Above patch is working around a false positive.

thanks,
suresh

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