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

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Message-ID: <20171107085107.GH3326@worktop>
Date:   Tue, 7 Nov 2017 09:51:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fengguang Wu <fengguang.wu@...el.com>
Cc:     Al Viro <viro@...IV.linux.org.uk>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <mszeredi@...e.cz>, lkp@...org,
        Sasha Levin <alexander.levin@...izon.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Byungchul Park <byungchul.park@....com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [d_alloc_parallel] WARNING: bad unlock balance detected!

On Tue, Nov 07, 2017 at 12:03:29PM +0800, Fengguang Wu wrote:

> >>[  428.512005] e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX
> >>LKP: HOSTNAME vm-lkp-wsx03-openwrt-i386-8, MAC , kernel 4.14.0-rc8 158, serial console /dev/ttyS0
> >>[  429.798345] Kernel tests: Boot OK!
> >>[  430.761760] [  430.766166] =====================================
> >>[  430.775297] WARNING: bad unlock balance detected!
> >>[  430.784342] 4.14.0-rc8 #158 Not tainted
> >>[  430.792153] -------------------------------------
> >>[  430.801319] pidof/1024 is trying to release lock (rcu_preempt_state) at:
> >>[  430.813514] [<c10e4348>] rcu_read_unlock_special+0x5f8/0x620
> >>[  430.824041] but there are no more locks to release!
> >
> >Er... yes?  What of that?  Since when is rcu_read_lock() not allowed to
> >be used under an rwsem?

That's not what it says, it is.. The warning is about trying to release
a lock that's not held. And its right, RCU was doing that. It would
acquire a lock without lockdep knowing about it and then telling lockdep
about freeing it.

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.