linux-kernel - Re: [PATCH 2/5] HWPOISON: fix tasklist_lock/anon

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Message-ID: <20090612100744.GB13607@wotan.suse.de>
Date:	Fri, 12 Jun 2009 12:07:44 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	"riel@...hat.com" <riel@...hat.com>,
	"chris.mason@...cle.com" <chris.mason@...cle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 2/5] HWPOISON: fix tasklist_lock/anon_vma locking order

On Fri, Jun 12, 2009 at 12:03:08PM +0200, Andi Kleen wrote:
> On Thu, Jun 11, 2009 at 10:22:41PM +0800, Wu Fengguang wrote:
> > To avoid possible deadlock. Proposed by Nick Piggin:
> 
> I disagree with the description. There's no possible deadlock right now.
> It would be purely out of paranoia.
> 
> > 
> >   You have tasklist_lock(R) nesting outside i_mmap_lock, and inside anon_vma
> >   lock. And anon_vma lock nests inside i_mmap_lock.
> > 
> >   This seems fragile. If rwlocks ever become FIFO or tasklist_lock changes
> 
> I was a bit dubious on this reasoning. If rwlocks become FIFO a lot of
> stuff will likely break.
> 
> >   type (maybe -rt kernels do it), then you could have a task holding
> 
> I think they tried but backed off quickly again
> 
> It's ok with a less scare-mongering description.

There's simply no good reason to invert ordering of locks like
this.

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