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: <20190501185400.GQ7905@worktop.programming.kicks-ass.net>
Date:   Wed, 1 May 2019 20:54:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...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>,
        Oleg Nesterov <oleg@...hat.com>, jack@...e.com,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) !=
 current) with fsfreeze (4.19.25-rt16)

On Wed, May 01, 2019 at 01:26:08PM -0400, Waiman Long wrote:
> On 5/1/19 1:09 PM, Peter Zijlstra wrote:
> > On Tue, Apr 30, 2019 at 03:28:11PM +0200, Peter Zijlstra wrote:
> >
> >> 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?
> 
> That code is in just in the tip tree. It is not in the mainline yet. I
> do realize that it can be a problem and so I have it modified to the
> following in my part2 patchset.

Nah, the percpu_rwsem abuse by the freezer is atrocious, we really
should not encourage that. Also, it completely wrecks -RT.

Hence the proposed patch.