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

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Date:	Mon, 30 Sep 2013 12:03:16 -0400
From:	Waiman Long <waiman.long@...com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Michel Lespinasse <walken@...gle.com>,
	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>,
	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 09/30/2013 03:05 AM, Ingo Molnar wrote:
> * Michel Lespinasse<walken@...gle.com>  wrote:
>
>> That said, I am very scared of using rwlock_t here, and I would much
>> prefer we choose a fair lock (either spinlock or a new rwlock
>> implementation which guarantees not to starve any locker thread)
> Given how few users rwlock_t has today we could attempt to make it
> reader-writer fair, as long as the usecase of a hardirq or softirq
> context always getting nested read access on the same CPU is preserved.
>
> (but that can be done - if nothing else then with an explicit context
> check.)
>
> Thanks,
>
> 	Ingo

My queue rwlock patch is able to handle the use case of a hardirq and 
softirq context and can be made almost as fair as the ticket spinlock.

-Longman
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