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Date: Wed, 23 Sep 2015 11:06:32 +0300
From: Vladimir Davydov <vdavydov@parallels.com>
To: David Rientjes <rientjes@google.com>
CC: Andrew Morton <akpm@linux-foundation.org>, Oleg Nesterov <oleg@redhat.com>,
        Michal Hocko <mhocko@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Stanislav Kozina <skozina@redhat.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Subject: Re: [patch] mm, oom: remove task_lock protecting comm printing
Message-ID: <20150923080632.GD12318@esperanza>
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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?

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'

Thanks,
Vladimir