linux-kernel - Re: [PATCH v2 9/8] don't fool lockdep in freeze_super() and thaw

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Message-ID: <20150813135838.GA17414@redhat.com>
Date:	Thu, 13 Aug 2015 15:58:38 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	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 9/8] don't fool lockdep in freeze_super() and
	thaw_super() paths

On 08/13, Jan Kara wrote:
>
> On Wed 12-08-15 15:11:38, Oleg Nesterov wrote:
> > On 08/11, Oleg Nesterov wrote:
> > >
> > > The only essential change is that I dropped the lockdep improvements
> > > as we discussed. This means that 8/8 was changed a bit, and I decided
> > > to add the new documentation patch, see 3/8.
> >
> > Update: The recent
> >
> > 	[PATCH 0/2] xfs: kill lockdep false positives from readdir
> >
> > changes from Dave fixed the problems ILOCK false-positives. So we can
> > add the additional patch which (modulo comments) just turns v2 back into
> > v1.
> >
> > Dave, Jan, you seem to agree with these patches. How should we route
> > this all?
>
> Regarding the routing, ideally Al Viro should take these as a VFS
> maintainer.

OK. I'll send v3.

But to remind, this particular patch depends on Dave's fixes, so I will
send it later.

And I forgot to mention that I have another patch which removes the
trylock hack from __sb_start_write() as Dave suggested, it passed the
tests. But again, I'd really like to send it separately so that it can
be reverted in (unlikely) case something else does recursive read_lock().

> > Subject: [PATCH v2 9/8] don't fool lockdep in freeze_super() and thaw_super() paths
> >
> > sb_wait_write()->percpu_rwsem_release() fools lockdep to avoid the
> > false-positives. Now that xfs was fixed by Dave we can remove it and
> > change freeze_super() and thaw_super() to run with s_writers.rw_sem
> > locks held; we add two trivial helpers for that, sb_freeze_release()
> > and sb_freeze_acquire().
> >
> > While at it, kill the outdated part of the comment above sb_wait_write.
> >
> > Signed-off-by: Oleg Nesterov <oleg@...hat.com>
>
> The patch looks good. Just one nit:
>
> > +	for (level = SB_FREEZE_LEVELS; --level >= 0; )
> > +		percpu_rwsem_release(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
>
> It is more common (and to me more readable) to have the loop written as:
>
> for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
>
> I agree what you do is shorter but IMHO it's just an unnecessary
> obfuscation :)

Agreed, will fix.

Oleg.

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