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 S1755331AbcHDFBo (ORCPT <rfc822;w@1wt.eu>);
	Thu, 4 Aug 2016 01:01:44 -0400
Received: from mail-ua0-f180.google.com ([209.85.217.180]:33400 "EHLO
	mail-ua0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751138AbcHDFBm (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 4 Aug 2016 01:01:42 -0400
MIME-Version: 1.0
In-Reply-To: <alpine.DEB.2.20.1608031330000.14875@east.gentwo.org>
References: <1469135662-31512-1-git-send-email-mathieu.desnoyers@efficios.com>
 <1469135662-31512-2-git-send-email-mathieu.desnoyers@efficios.com>
 <CALCETrVq3hgLbADcx_4g6gGgp7Hf-WnnWA8gg68tPZTv2wfbDg@mail.gmail.com>
 <1806206514.82247.1469502139408.JavaMail.zimbra@efficios.com>
 <20160803122717.GL6862@twins.programming.kicks-ass.net> <CALCETrWw0M9YLWTR7_ruxY8cW9_5XRONdw1QN+_pBp0Y63L8pA@mail.gmail.com>
 <alpine.DEB.2.20.1608031330000.14875@east.gentwo.org>
From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 3 Aug 2016 22:01:20 -0700
Message-ID: <CALCETrXiyFRT_Fqvq2jMpUAWzJ2h9hcAAuGVe4ekFXb5_Z5NEg@mail.gmail.com>
Subject: Re: [RFC PATCH v7 1/7] Restartable sequences system call
To: Christoph Lameter <cl@linux.com>
Cc: Ben Maurer <bmaurer@fb.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Russell King <linux@arm.linux.org.uk>,
        linux-api <linux-api@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Dave Watson <davejwatson@fb.com>, rostedt <rostedt@goodmis.org>,
        Will Deacon <will.deacon@arm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andi Kleen <andi@firstfloor.org>,
        Josh Triplett <josh@joshtriplett.org>, Paul Turner <pjt@google.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Andrew Hunter <ahh@google.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Aug 3, 2016 11:31 AM, "Christoph Lameter" <cl@linux.com> wrote:
>
> On Wed, 3 Aug 2016, Andy Lutomirski wrote:
>
> > > Well, a CMPXCHG without LOCK prefix isn't all that expensive on x86.
> > >
> > > It is however on PPC and possibly other architectures, so in name of
> > > simplicity supporting only the one variant makes sense.
> > >
> >
> > I wouldn't want to depend on CMPXCHG.  But imagine we had primitives
> > that were narrower than the full abort-on-preemption primitive.
> > Specifically, suppose we had abort if (actual cpu != expected_cpu ||
> > *aptr != aval).  We could do things like:
> >
>
> The latency issues that are addressed by restartable sequences require
> minimim instruction overhead. Lockless CMPXCHG is very important in that
> area and I would not simply remove it from consideration.

What I mean is: I think the solution shouldn't depend on the
x86-specific unlocked CMPXCHG instruction if it can be avoided.