linux-kernel - Re: [PATCH v2 7/8] shift percpu_counter_destroy() into destroy_super

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Date:	Thu, 13 Aug 2015 16:09:29 +0200
From:	Jan Kara <jack@...e.cz>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 7/8] shift percpu_counter_destroy() into
 destroy_super_work()

On Thu 13-08-15 15:36:16, Oleg Nesterov wrote:
> On 08/13, Jan Kara wrote:
> >
> > On Tue 11-08-15 19:04:16, Oleg Nesterov wrote:
> > >
> > > So this is just the temporary kludge which helps us to avoid the
> > > conflicts with the changes which will be (hopefully) routed via
> > > rcu tree.
> > >
> > > Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> >
> > Looking into this again, it would seem somewhat cleaner to me to move the
> > destruction to deactivate_locked_super() instead.
> 
> Heh ;) You know, I was looking at deactivate_locked_super(). However, I
> simply do not understand this code enough, I failed to verify it would
> be safe to destroy s_writers there.

Yes, it will be safe. After ->kill_sb() callback the filesystem is dead.
There can be someone still holding reference to superblock but these are
just users inspecting the structure definitely not caring about freeze
protection.

> And. Please note destroy_super() in alloc_super() error path, so this
> needs a bit more changes in any case.

Yes. But you can sleep in alloc_super() so that would be easy enough.

> Can't we live with this hack for now? To remind, it will be reverted
> (at least partially) in any case. Yes, yes, it is very ugly and the
> changelog documents this fact. But it looks simple and safe. To me
> it would be better to make the conversion first, then cleanup this
> horror after another discussion.

All I care about is that long-term, all handling from destroy_super() that
needs to sleep ends up in one place. So if you promise you'll make this
happen I can live with the workqueue solution for now (but you have to
convince also Al as a maintainer ;).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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