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Date:   Fri, 3 May 2019 16:16:33 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        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>, jack@suse.com,
        Waiman Long <longman@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) !=
 current) with fsfreeze (4.19.25-rt16)
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On Thu, May 02, 2019 at 12:09:32PM +0200, Oleg Nesterov wrote:
> On 05/01, Peter Zijlstra wrote:
> >
> > Anyway; I cobbled together the below. Oleg, could you have a look, I'm
> > sure I messed it up.
> 
> Oh, I will need to read this carefully. but at first glance I do not see
> any hole...
> 
> > +static void readers_block(struct percpu_rw_semaphore *sem)
> > +{
> > +	wait_event_cmd(sem->writer, !sem->readers_block,
> > +		       __up_read(&sem->rw_sem), __down_read(&sem->rw_sem));
> > +}
> > +
> > +static void block_readers(struct percpu_rw_semaphore *sem)
> > +{
> > +	wait_event_exclusive_cmd(sem->writer, !sem->readers_block,
> > +				 __up_write(&sem->rw_sem),
> > +				 __down_write(&sem->rw_sem));
> > +	/*
> > +	 * Notify new readers to block; up until now, and thus throughout the
> > +	 * longish rcu_sync_enter() above, new readers could still come in.
> > +	 */
> > +	WRITE_ONCE(sem->readers_block, 1);
> > +}
> 
> So iiuc, despite it name block_readers() also serializes the writers, ->rw_sem
> can be dropped by down_write_non_owner() so the new writer can take this lock.

I don't think block_readers() is sufficient to serialize writers;
suppose two concurrent callers when !->readers_block. Without ->rwsem
that case would not serialize.

> But this all is cosmetic, it seems that we can remove ->rw_sem altogether
> but I am not sure...

Only if we introduce something like ->wait_lock to serialize things.