linux-kernel - Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16)

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Message-ID: <20190430142950.GG2589@hirez.programming.kicks-ass.net>
Date:   Tue, 30 Apr 2019 16:29:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>, jack@...e.com
Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) !=
 current) with fsfreeze (4.19.25-rt16)

On Tue, Apr 30, 2019 at 04:15:01PM +0200, Oleg Nesterov wrote:
> Sorry, I don't understand...

So the problem is that on -RT rwsem uses PI mutexes, and the below just
cannot work.

Also see:

  https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/include/linux/rwsem_rt.h?h=linux-5.0.y-rt
  https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/kernel/locking/rwsem-rt.c?h=linux-5.0.y-rt

> On 04/30, Peter Zijlstra wrote:
> >
> > Thaw then does the reverse, frobs lockdep
> 
> Yes, in particular it does
> 
> 	lockdep_sb_freeze_acquire()
> 		percpu_rwsem_acquire()
> 			sem->rw_sem.owner = current;
> 

Oh ARGGH, I missed that. This is horrible :-(