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Date:   Tue, 30 Apr 2019 15:45:48 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-rt-users <linux-rt-users@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Clark Williams <williams@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Oleg Nesterov <oleg@redhat.com>, jack@suse.com
Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) !=
 current) with fsfreeze (4.19.25-rt16)
Message-ID: <20190430134547.lll7kjrslh3zat2b@linutronix.de>
References: <20190326093421.GA29508@localhost.localdomain>
 <20190419085627.GI4742@localhost.localdomain>
 <20190430125130.uw7mhdnsoqr2v3gf@linutronix.de>
 <20190430132811.GB2589@hirez.programming.kicks-ass.net>
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On 2019-04-30 15:28:11 [+0200], Peter Zijlstra wrote:
> On Tue, Apr 30, 2019 at 02:51:31PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2019-04-19 10:56:27 [+0200], Juri Lelli wrote:
> > > On 26/03/19 10:34, Juri Lelli wrote:
> > > > Hi,
> > > > 
> > > > Running this reproducer on a 4.19.25-rt16 kernel (with lock debugging
> > > > turned on) produces warning below.
> > > 
> > > And I now think this might lead to an actual crash.
> > 
> > Peter, could you please take a look at the thread:
> >   https://lkml.kernel.org/r/20190419085627.GI4742@localhost.localdomain
> > 
> > I assumed that returning to userland with acquired locks is something we
> > did not want…
> 
> Yeah, but AFAIK fs freezing code has a history of doing exactly that..
> This is just the latest incarnation here.
> 
> So the immediate problem here is that the task doing thaw isn't the same
> that did freeze, right? The thing is, I'm not seeing how that isn't a
> problem with upstream either.
> 
> The freeze code seems to do: percpu_down_write() for the various states,
> and then frobs lockdep state.
> 
> Thaw then does the reverse, frobs lockdep and then does: percpu_up_write().
> 
> percpu_down_write() directly relies on down_write(), and
> percpu_up_write() on up_write(). And note how __up_write() has:
> 
> 	DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
> 
> So why isn't this same code coming unstuck in mainline?

I have to re-route most of this questions to Juri Lelli.
Lockdep has these gems:
	lockdep_sb_freeze_release() / lockdep_sb_freeze_acquire()

Sebastian