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From: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Date: Tue, 16 Oct 2012 21:53:02 -0400
Message-ID: <CAHGf_=pVx=7nCZ1+e2M+bKH=SUGJa585+QxFbNboaq7_vy4MVw@mail.gmail.com>
Subject: Re: [patch for-3.7] mm, mempolicy: fix printing stack contents in numa_maps
To: David Rientjes <rientjes@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Jones <davej@redhat.com>, bhutchings@solarflare.com,
        Konstantin Khlebnikov <khlebnikov@openvz.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Hugh Dickins <hughd@google.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Tue, Oct 16, 2012 at 9:49 PM, David Rientjes <rientjes@google.com> wrote:
> 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.

I didn't say old comment is true. I just only your new comment also false.
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