linux-kernel - Re: [PATCH] mm: protect against concurrent vma expansion

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Message-ID: <20130104024935.GF4939@ZenIV.linux.org.uk>
Date:	Fri, 4 Jan 2013 02:49:36 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Simon Jeons <simon.jeons@...il.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: protect against concurrent vma expansion

On Thu, Jan 03, 2013 at 06:40:05PM -0600, Simon Jeons wrote:
> On Wed, 2012-12-19 at 19:01 -0800, Michel Lespinasse wrote:
> > Hi Simon,
> > 
> > On Wed, Dec 19, 2012 at 5:56 PM, Simon Jeons <simon.jeons@...il.com> wrote:
> > > One question.
> > >
> > > I found that mainly callsite of expand_stack() is #PF, but it holds
> > > mmap_sem each time before call expand_stack(), how can hold a *shared*
> > > mmap_sem happen?
> > 
> > the #PF handler calls down_read(&mm->mmap_sem) before calling expand_stack.
> > 
> > I think I'm just confusing you with my terminology; shared lock ==
> > read lock == several readers might hold it at once (I'd say they share
> > it)
> 
> Sorry for my late response. 
> 
> Since expand_stack() will modify vma, then why hold a read lock here?

To prevent that vma being ripped out.
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