From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S932688AbaAaTqV (ORCPT <rfc822;w@1wt.eu>);
	Fri, 31 Jan 2014 14:46:21 -0500
Received: from merlin.infradead.org ([205.233.59.134]:54248 "EHLO
	merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754033AbaAaTqT (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 31 Jan 2014 14:46:19 -0500
Date: Fri, 31 Jan 2014 20:45:35 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hp.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
        linux-arch@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michel Lespinasse <walken@google.com>,
        Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        George Spelvin <linux@horizon.com>,
        Daniel J Blueman <daniel@numascale.com>,
        Alexander Fyodorov <halcy@yandex.ru>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>,
        Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@hp.com>
Subject: Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock
 implementation
Message-ID: <20140131194535.GN5002@laptop.programming.kicks-ass.net>
References: <1390933151-1797-1-git-send-email-Waiman.Long@hp.com>
 <1390933151-1797-2-git-send-email-Waiman.Long@hp.com>
 <1391108430.3138.86.camel@schen9-DESK>
 <20140130192835.GK5002@laptop.programming.kicks-ass.net>
 <52EBEAD5.3000502@hp.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <52EBEAD5.3000502@hp.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Jan 31, 2014 at 01:26:29PM -0500, Waiman Long wrote:
> >I don't get why we need the used thing at all; something like:
> >
> >struct qna {
> >	int cnt;
> >	struct qnode nodes[4];
> >};
> >
> >DEFINE_PER_CPU(struct qna, qna);
> >
> >struct qnode *get_qnode(void)
> >{
> >	struct qna *qna = this_cpu_ptr(&qna);
> >
> >	return qna->nodes[qna->cnt++]; /* RMW */
> >}
> >
> >void put_qnode(struct qnode *qnode)
> >{
> >	struct qna *qna = this_cpu_ptr(&qna);
> >	qna->cnt--;
> >}
> >
> >Should do fine, right?
> 
> Yes, we can do something like that. However I think put_qnode() needs to use
> atomic dec as well. As a result, we will need 2 additional atomic operations
> per slowpath invocation. The code may look simpler, but I don't think it
> will be faster than what I am currently doing as the cases where the used
> flag is set will be relatively rare.

No, put doesn't need an atomic; nor is it as well; because the inc
doesn't need an atomic either.

> >If we interrupt the RMW above the interrupted context hasn't yet used
> >the queue and once we return its free again, so all should be well even
> >on load-store archs.
> >
> >The nodes array might as well be 3, because NMIs should never contend on
> >a spinlock, so all we're left with is task, softirq and hardirq context.
> 
> I am not so sure about NMI not taking a spinlock. I seem to remember seeing
> code that did that. Actually, I think the NMI code is trying to printk
> something which, in turn, need to acquire a spinlock.

Yeah I know, terribly broken that, I've been waiting for that to explode
:-)