Re: [PATCH v2 7/8] shift percpu_counter_destroy() into destroy_super_work()

From: Oleg Nesterov <oleg@redhat.com>
To: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Dave Chinner <david@fromorbit.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 7/8] shift percpu_counter_destroy() into destroy_super_work()
Date: Thu, 13 Aug 2015 17:20:35 +0200	[thread overview]
Message-ID: <20150813152035.GB20045@redhat.com> (raw)
In-Reply-To: <20150813140929.GA4392@quack.suse.cz>

On 08/13, Jan Kara wrote:
>
> On Thu 13-08-15 15:36:16, Oleg Nesterov wrote:
> > On 08/13, Jan Kara wrote:
> > >
> > > 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.

OK, thanks.

> > 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.

Yes, yes, I didn't mean this is a problem.

> > 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

I certainly promise I will try to do something in any case ;)

But let me repeat another reason why I think we should do this later.
The necessary changes depend on other work-in-progress rcu_sync changes
in percpu_rw_semaphore.

Now that you confirm that we should not worry about sb_writers after
deactivate_locked_super(), the cleanup looks even simpler than I
thought initially:

	1. We do not even need to destroy the counters in
	   deactivate_locked_super(). It should only stop the
	   (potentially) pending rcu-callback(s).

	2. Just revert this patch altogether.

> (but you have to
> convince also Al as a maintainer ;).

Perhaps he won't notice how ugly this change is? If you won't tell him.

Oleg.