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

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Date:	Mon, 30 Sep 2013 11:58:29 -0400
From:	Waiman Long <waiman.long@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...nel.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

On 09/28/2013 03:33 PM, Linus Torvalds wrote:
> On Sat, Sep 28, 2013 at 12:21 PM, Ingo Molnar<mingo@...nel.org>  wrote:
>> If we do that then I suspect the next step will be queued rwlocks :-/ The
>> current rwlock_t implementation is rather primitive by modern standards.
>> (We'd probably have killed rwlock_t long ago if not for the
>> tasklist_lock.)
> Yeah, I'm not happy about or rwlocks. That's one lock that currently
> is so broken that I think we could easily argue for making that one
> queued.
>
> Waiman had a qrwlock series that looked reasonable, and I think his
> later versions were drop-in replacements (ie they automatically just
> did the RightThing(tm) wrt interrupts taking a recursive read lock - I
> objected to the first versions that required that to be stated
> explicitly).

The latest version (v3) will allow recursive read lock in interrupt 
handlers.

> 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.

-Longman
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