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

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Date:	Thu, 16 Jul 2015 09:26:54 +0200
From:	Jan Kara <jack@...e.cz>
To:	Oleg Nesterov <oleg@...hat.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 Wed 15-07-15 20:19:20, Oleg Nesterov wrote:
> On 07/15, Jan Kara wrote:
> > So I'm also in favor of Oleg's approach
> > as well. We just have to wait until he fixes the outstanding issues with
> > his code.
> 
> Yes. I'll try to make the working version, hopefully this week.
> 
> But,
> 
> > Dave, just send your fsnotify patch separately to AKPM - he
> > usually merges fsnotify stuff.
> 
> Perhaps it makes to merge other 2 patches from Dave first? (those which
> change __sb_start/end_write to rely on RCU). Afaics these changes are
> straightforward and correct. Although I'd suggest to use preempt_disable()
> and synchronize_sched() instead. I will be happy to (try to) make this
> conversion on top of his changes.
> 
> Because I do not want to delay the performance improvements and I do not
> know when exactly I'll send the next version: I need to finish the previous
> discussion about rcu_sync first. And the necessary changes in fs/super.c
> depend on whether percpu_rw_semaphore will have rcu_sync or not (not too
> much, only destroy_super() depends, but still).
> 
> And of course, I am worried that I missed something and percpu_rw_semaphore
> can't work for some reason. The code in fs/super.c looks simple, but it
> seems that filesystems do the "strange" things with lockdep at least.

So Dave's patches would go in only in the next merge window anyway so we
still have like two-three weeks to decide which patchset to take. If you
think it will take you longer, then merging Dave's patches makes some sense
although I personally don't think the issue is so important that we have to
fix it ASAP and eventual delay of one more release would be OK for me.

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