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Date: Sat, 18 Jul 2015 01:29:30 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Daniel Wagner <daniel.wagner@bmw-carit.de>,
        Davidlohr Bueso <dave@stgolabs.net>, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/7] Add rcu_sync infrastructure to avoid _expedited()
	in percpu-rwsem
Message-ID: <20150717232930.GA16247@redhat.com>
References: <20150711233535.GA829@redhat.com> <20150715182713.GL3717@linux.vnet.ibm.com> <20150715193601.GA4386@redhat.com> <20150715215927.GO3717@linux.vnet.ibm.com>
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On 07/15, Paul E. McKenney wrote:
>
> On Wed, Jul 15, 2015 at 09:36:01PM +0200, Oleg Nesterov wrote:
> >
> > Do you mean you need another user except percpu_rw_semaphore? I do
> > not see any right now...
>
> Not asking for more than one use, but it does need a use.  I believe
> that percpu_rw_semaphore suffices.
>
> > Let me remind about sb_writers again. It actually has 3 rw_sem's
> > and I am trying to turn then into percpu_rw_semaphore's.
> >
> > In this case freeze_super() will need 6 synchronize_sched_expedited().
> > This just looks ugly. But if we have rcu_sync primitives, all 3 sem's
> > in struct super_block can share the same "struct rcu_sync", and
> > freeze_super() will need only once synchronize_sched().
>
> Makes sense.

Great, thanks. And iiuc Linus doesn't object to this particular change.
Plus I see the "Make checkpatch.pl warn on expedited RCU grace periods"
patch ;)

So can I assume you will take these changes?

I do not need them right now, just I need to know what should I do in
destroy_super() and (much more importantly) what should I say in the
changelogs if I try to convert sb_writers to use percpu_rw_semaphore.

Oleg.