linux-kernel - Re: [patch for-3.7] mm, mempolicy: fix printing stack contents in numa

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Message-ID: <alpine.DEB.2.00.1210161847060.20503@chino.kir.corp.google.com>
Date:	Tue, 16 Oct 2012 18:49:00 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, bhutchings@...arflare.com,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch for-3.7] mm, mempolicy: fix printing stack contents in
 numa_maps

On Tue, 16 Oct 2012, KOSAKI Motohiro wrote:

> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 0b78fb9..d04a8a5 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -1536,9 +1536,8 @@ asmlinkage long compat_sys_mbind(compat_ulong_t start, compat_ulong_t len,
> >   *
> >   * Returns effective policy for a VMA at specified address.
> >   * Falls back to @task or system default policy, as necessary.
> > - * Current or other task's task mempolicy and non-shared vma policies
> > - * are protected by the task's mmap_sem, which must be held for read by
> > - * the caller.
> > + * Current or other task's task mempolicy and non-shared vma policies must be
> > + * protected by task_lock(task) by the caller.
> 
> This is not correct. mmap_sem is needed for protecting vma. task_lock()
> is needed to close vs exit race only when task != current. In other word,
> caller must held both mmap_sem and task_lock if task != current.
> 

The comment is specifically addressing non-shared vma policies, you do not 
need to hold mmap_sem to access another thread's mempolicy.
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