linux-kernel - Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw

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Message-ID: <20150720162353.GA1897@redhat.com>
Date:	Mon, 20 Jul 2015 18:23:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jan Kara <jack@...e.cz>, Dave Hansen <dave.hansen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw_semaphore

On 07/18, Dave Chinner wrote:
>
> On Fri, Jul 17, 2015 at 07:31:17PM +0200, Oleg Nesterov wrote:
> >
> > Dave, I didn't write this comment. Please look at acquire_freeze_lock().
> > If we can remove this logic - great! but this needs a separate change.
>
> Oh, I think I know what it was - when we duplicate a transaction for
> a rolling commit, we do it before committing the current transaction
> is committed. I *think* that used to take a second freeze reference,
> which only existed until the first transaction was committed. We do
> things a bit differently now - we hold a state flag on the
> transaction to indicate it needs to release the freeze reference
> when it is freed and we pass it to the new transaction so that the
> first transaction commit doesn't release it.

Just fyi, please do not assume I can understand the explanation above ;)

> So, yes, it may well be a stale comment now.

Perhaps. But this needs a separate change. Plus even if we remove this
hack, this code has other problems with lockdep.

I'll send the "lockdep" fixes/cleanup today, please review.

Oleg.

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