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Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current)
 with fsfreeze (4.19.25-rt16)
To:     Peter Zijlstra <peterz@infradead.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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,
        Davidlohr Bueso <dave@stgolabs.net>
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <ce854251-139e-15f1-2ac5-b66a27f8284c@redhat.com>
Date:   Wed, 1 May 2019 13:26:08 -0400
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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.

static inline void __up_write(struct rw_semaphore *sem)
{
        long tmp;

        /*
         * sem->owner may differ from current if the ownership is
transferred
         * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
         */
        DEBUG_RWSEMS_WARN_ON((sem->owner != current) &&
                            !((long)sem->owner & RWSEM_NONSPINNABLE), sem);

Maybe I should break this part out and have it merged into tip as well.

Cheers,
Longman