linux-kernel - Re: [RFC][PATCH 0/5] Optimize percpu-rwsem

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Message-ID: <20150527092839.GA12011@opentech.at>
Date:	Wed, 27 May 2015 11:28:39 +0200
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 0/5] Optimize percpu-rwsem

On Tue, 26 May 2015, Linus Torvalds wrote:

> On Tue, May 26, 2015 at 11:42 AM, Davidlohr Bueso <dave@...olabs.net> wrote:
> >
> > Instead of dropping percpu-rwsem, I was thinking we could instead look
> > for opportunities to convert new users, for instance shinkers, where the
> > write lock is also taken just for register and unregister purposes,
> > similar to uprobes.
> 
> So if there really are useful use cases for this, I don't object to
> the patch. It seems to just improve on a currently very low-usage
> locking primitive.
> 
> And it's not like I conceptually mind the notion of a percpu rwsem, I
> just hate seeing specialty locking that isn't really worth it.
> 
> Because as it is, with the current single use, I don't think it's even
> worth improving on.
> 
> I _would_ ask that people who are looking at this also look at our
> "lglock" thing. It's pretty much *exactly* the same thing, except for
> spinlocks, and that one too has exactly two users (the documentation
> states that the only user is stop_machine, but in fact file locking
> does too).
>
not sure where this would be missing:
Documentation/locking/lglock.txt
"Users: currently only the VFS and stop_machine related code"

I atleast did not find any other users as of 3.18 and in 4.0-rc5 this
still seems valid.

thx!
hofrat
 
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