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Date: Wed, 23 Sep 2015 18:13:54 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [patch] mm, oom: remove task_lock protecting comm printing
Message-ID: <20150923091354.GA640@swordfish>
References: <alpine.DEB.2.10.1509221629440.7794@chino.kir.corp.google.com>
 <20150923080632.GD12318@esperanza>
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Sender: owner-linux-mm@kvack.org
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To: Vladimir Davydov <vdavydov@parallels.com>
Cc: David Rientjes <rientjes@google.com>, Andrew Morton <akpm@linux-foundation.org>, Oleg Nesterov <oleg@redhat.com>, Michal Hocko <mhocko@kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Kyle Walker <kwalker@redhat.com>, Christoph Lameter <cl@linux.com>, Johannes Weiner <hannes@cmpxchg.org>, linux-mm <linux-mm@kvack.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Stanislav Kozina <skozina@redhat.com>, Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>

On (09/23/15 11:06), Vladimir Davydov wrote:
> Hi,
> 
> On Tue, Sep 22, 2015 at 04:30:13PM -0700, David Rientjes wrote:
> > The oom killer takes task_lock() in a couple of places solely to protect
> > printing the task's comm.
> > 
> > A process's comm, including current's comm, may change due to
> > /proc/pid/comm or PR_SET_NAME.
> > 
> > The comm will always be NULL-terminated, so the worst race scenario would
> > only be during update.  We can tolerate a comm being printed that is in
> > the middle of an update to avoid taking the lock.
> > 
> > Other locations in the kernel have already dropped task_lock() when
> > printing comm, so this is consistent.
> 
> Without the protection, can't reading task->comm race with PR_SET_NAME
> as described below?

the previous name was already null terminated, so it should be

	[name\0old_name\0]

	-ss

> 
> Let T->comm[16] = "name\0rubbish1234"
> 
> CPU1                                    CPU2
> ----                                    ----
> set_task_comm(T, "longname\0")
>   T->comm[0] = 'l'
>   T->comm[1] = 'o'
>   T->comm[2] = 'n'
>   T->comm[3] = 'g'
>   T->comm[4] = 'n'
>                                         printk("%s\n", T->comm)
>                                           T->comm = "longnrubbish1234"
>                                           OOPS: the string is not
>                                                 nil-terminated!
>   T->comm[5] = 'a'
>   T->comm[6] = 'm'
>   T->comm[7] = 'e'
>   T->comm[8] = '\0'

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