linux-kernel - Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

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Message-ID: <20131001073301.GA20889@gmail.com>
Date:	Tue, 1 Oct 2013 09:33:01 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Alex Shi <alex.shi@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path


* Waiman Long <waiman.long@...com> wrote:

> > I think Waiman's patches (even the later ones) made the queued rwlocks 
> > be a side-by-side implementation with the old rwlocks, and I think 
> > that was just being unnecessarily careful. It might be useful for 
> > testing to have a config option to switch between the two, but we 
> > might as well go all the way.
> 
> It is not actually a side-by-side implementation. A user can choose 
> either regular rwlock or the queue one, but never both by setting a 
> configuration parameter. However, I now think that maybe we should do it 
> the lockref way by pre-determining it on a per-architecture level 
> without user visible configuration option.

Well, as I pointed it out to you during review, such a Kconfig driven 
locking API choice is a no-go!

What I suggested instead: there's absolutely no problem with providing a 
better rwlock_t implementation, backed with numbers and all that.

Thanks,

	Ingo
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