linux-kernel - Re: [copy_namespaces] WARNING: bad unlock balance detected!

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Date:   Tue, 7 Nov 2017 20:29:57 +0800
From:   Fengguang Wu <fengguang.wu@...el.com>
To:     linux-kernel@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Emelyanov <xemul@...nvz.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Cedric Le Goater <clg@...ibm.com>,
        "Serge E. Hallyn" <serue@...ibm.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [copy_namespaces] WARNING: bad unlock balance detected!

Sorry please ignore this report -- according to Peter:

This is fixed by commit:

  02a7c234e540 ("rcu: Suppress lockdep false-positive ->boost_mtx complaints")

The problem is that RCU boosting was mixing futex and !futex rt_mutex
ops.

On Tue, Nov 07, 2017 at 09:59:01AM +0800, Fengguang Wu wrote:
>Hi,
>
>Here is a warning in v4.14-rc8 -- it's not necessarily a new bug.
>
>[  266.395343] rcu-torture: rcu_torture_fakewriter task started
>[  266.396862] rcu-torture: Creating rcu_torture_reader task
>[  266.416970] rcu-torture: Creating rcu_torture_stats task
>[  266.458705]
>[  266.459151] =====================================
>[  266.459806] WARNING: bad unlock balance detected!
>[  266.462394] 4.14.0-rc8 #166 Not tainted
>[  266.462394] -------------------------------------
>[  266.462394] kthreadd/2 is trying to release lock (rcu_preempt_state) at:
>[  266.462394] [<810a96e7>] rcu_read_unlock_special+0x267/0x2d0
>[  266.462394] but there are no more locks to release!
>[  266.462394]
>[  266.462394] other info that might help us debug this:
>[  266.462394] 1 lock held by kthreadd/2:
>[  266.462394]  #0:  (rcu_read_lock){....}, at: [<810606f0>] copy_namespaces+0x0/0x180
>[  266.462394]
>[  266.462394] stack backtrace:
>[  266.462394] CPU: 0 PID: 2 Comm: kthreadd Not tainted 4.14.0-rc8 #166
>[  266.462394] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
>[  266.462394] Call Trace:
>[  266.462394]  ? dump_stack+0x74/0xab
>[  266.462394]  ? rcu_read_unlock_special+0x267/0x2d0
>[  266.462394]  ? print_unlock_imbalance_bug+0xb4/0xc0
>[  266.462394]  ? rcu_read_unlock_special+0x267/0x2d0
>[  266.462394]  ? lock_release+0x39b/0x3e0
>[  266.462394]  ? rt_mutex_unlock+0x18/0x80
>[  266.462394]  ? rcu_read_unlock_special+0x267/0x2d0
>[  266.462394]  ? __rcu_read_unlock+0x3b/0x60
>[  266.462394]  ? copy_namespaces+0xf5/0x180
>[  266.462394]  ? copy_process+0x15f0/0x1670
>[  266.462394]  ? kthread_stop+0x50/0x50
>[  266.462394]  ? kthreadd+0xe5/0x140
>[  266.462394]  ? sched_clock+0x9/0x10
>[  266.462394]  ? kthreadd+0xe5/0x140
>[  266.462394]  ? sched_clock_local+0x1d/0x130
>[  266.462394]  ? kthread_stop+0x50/0x50
>[  266.462394]  ? _do_fork+0x76/0x480
>[  266.462394]  ? finish_task_switch+0x145/0x1c0
>[  266.462394]  ? find_held_lock+0x22/0x90
>[  266.462394]  ? kthread_stop+0x50/0x50
>[  266.462394]  ? kernel_thread+0x19/0x20
>[  266.462394]  ? kthreadd+0xf6/0x140
>[  266.462394]  ? kthread_create_on_cpu+0x80/0x80
>[  266.462394]  ? ret_from_fork+0x19/0x24
>[  266.537631] rcu-torture: rcu_torture_reader task started
>[  266.541600] rcu-torture: Creating torture_shuffle task
>[  266.544634] rcu-torture: rcu_torture_stats task started
>[  266.548208] rcu-torture: Creating torture_stutter task
>[  266.552156] rcu-torture: torture_shuffle task started
>
>Thanks,
>Fengguang