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

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Message-ID: <20150713231451.GI7943@dastard>
Date:	Tue, 14 Jul 2015 09:14:51 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	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, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw_semaphore

On Tue, Jul 14, 2015 at 12:42:37AM +0200, Oleg Nesterov wrote:
> On 07/14, Dave Chinner wrote:
> >
> > [ Please cc linux-fsdevel@...r.kernel.org on filesystem
> > infrastructure changes! ]
> 
> OK, will do.
> 
> > On Mon, Jul 13, 2015 at 11:25:36PM +0200, Oleg Nesterov wrote:
> > >
> > > 	- sb_lockdep_release() and sb_lockdep_acquire() play with
> > > 	  percpu_rw_semaphore's internals.
> > >
> > > 	  Trivial, we need a couple of new helper in percpu-rwsem.c.
> >
> > 	- try compiling XFS, watch it break on freeze lockdep
> > 	  annotations
> 
> Thanks a lot! I see. Still trivial, xfs can use the same helpers
> rather the abuse lockdep directly.
> 
> > > 	- Most probably I missed something else, and I do not need
> > > 	  how to test.
> >
> > xfstests has many freeze related stress tests.  IIRC, generic/068 is
> > the test that historically causes the most problems for freeze
> > infrastructure changes. You'll also need to test at least ext4, XFS
> > and btrfs, because they all stress the freeze code differently.
> > Testing XFS, in particular, is a good idea because it has several
> > custom freeze tests that aren't run on any other filesystem type.
> 
> Thanks again.
> 
> Do you see something fundamentally wrong with this change?

I haven't looked particularly closely at the implementation, just
enough to get an idea of the semantics of the new infrasructure (I
didn't know that per-cpu rwsems existed!). The freeze code is
essentially a multi-level read-optimised read/write barrier and
AFAICT the per-cpu rw-sem has those semantics. From that perspective
I don't see any fundamental problems, but there may be details that
I've missed....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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