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 S1752524AbbCPD0G (ORCPT <rfc822;w@1wt.eu>);
	Sun, 15 Mar 2015 23:26:06 -0400
Received: from relay6-d.mail.gandi.net ([217.70.183.198]:43128 "EHLO
	relay6-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1750937AbbCPD0D (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Sun, 15 Mar 2015 23:26:03 -0400
X-Originating-IP: 50.43.43.179
Date: Sun, 15 Mar 2015 20:25:50 -0700
From: Josh Triplett <josh@joshtriplett.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: linux-kernel@vger.kernel.org,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Nicholas Miell <nmiell@comcast.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Stephen Hemminger <stephen@networkplumber.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        David Howells <dhowells@redhat.com>, Nick Piggin <npiggin@kernel.dk>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier
 (x86) (v12)
Message-ID: <20150316032550.GA1110@thin>
References: <1426447459-28620-1-git-send-email-mathieu.desnoyers@efficios.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1426447459-28620-1-git-send-email-mathieu.desnoyers@efficios.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Sun, Mar 15, 2015 at 03:24:19PM -0400, Mathieu Desnoyers wrote:
> Here is an implementation of a new system call, sys_membarrier(), which
> executes a memory barrier on either all running threads of the current
> process (MEMBARRIER_PRIVATE_FLAG) or calls synchronize_sched() to issue
> a memory barrier on all threads running on the system. It can be used to
> distribute the cost of user-space memory barriers asymmetrically by
> transforming pairs of memory barriers into pairs consisting of
> sys_membarrier() and a compiler barrier. For synchronization primitives
> that distinguish between read-side and write-side (e.g. userspace RCU,
> rwlocks), the read-side can be accelerated significantly by moving the
> bulk of the memory barrier overhead to the write-side.

>>From a quick review, this seems quite reasonable (as it did 5 years
ago).

One request: Could you please add a config option (default y) in the
EXPERT menu to disable this?  You actually seem to already have it
marked as a cond_syscall.

Also, a very minor nit: flags in kernel APIs aren't typically named with
a _FLAG suffix.

With the syscall made optional, and with or without that naming nit
fixed:
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

- Josh Triplett