linux-kernel - BUG: cgroup_task_counter subsys may crash with whole-threadgroup move

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Message-ID: <20111014000913.GA22527@ghc17.ghc.andrew.cmu.edu>
Date:	Thu, 13 Oct 2011 20:09:13 -0400
From:	Ben Blum <bblum@...rew.cmu.edu>
To:	Frederic Weisbecker <fweisbec@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul Menage <paul@...lmenage.org>,
	linux-kernel@...r.kernel.org, bblum@...rew.cmu.edu
Subject: BUG: cgroup_task_counter subsys may crash with whole-threadgroup
 move

I was testing some patches for cgroup_attach_proc and managed to cause a
crash with the following usage pattern:

    mount -t cgroup none -o tasks /dev/cgroup
    cd /dev/cgroup
    mkdir foo
    echo $PID > foo/cgroup.procs
    echo $PID > tasks
    echo $PID > foo/cgroup.procs

Where $PID is the thread ID of a member of a multithreaded process (my
test program just does CLONE_THREAD 8 times and then all threads sleep).
(It doesn't matter if the thread is the group leader or not, but a
single-threaded process doesn't crash.)

And get the following kernel panic:
http://maximegalon.andrew.cmu.edu/cgroup-taskstats/panic.txt

It's deterministic, and happens only when the "tasks" subsystem is
mounted.

I'm using user-mode linux to test, with the following config:
http://maximegalon.andrew.cmu.edu/cgroup-taskstats/config.txt

and I ran it in GDB to get the following backtrace:
http://maximegalon.andrew.cmu.edu/cgroup-taskstats/bt.txt

Thanks,
Ben
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