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 S965207Ab2JZQK5 (ORCPT <rfc822;w@1wt.eu>);
	Fri, 26 Oct 2012 12:10:57 -0400
Received: from mx1.redhat.com ([209.132.183.28]:14771 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S964981Ab2JZQK4 (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Fri, 26 Oct 2012 12:10:56 -0400
Date: Fri, 26 Oct 2012 12:09:41 -0400 (EDT)
From: Mikulas Patocka <mpatocka@redhat.com>
X-X-Sender: mpatocka@file.rdu.redhat.com
To: Oleg Nesterov <oleg@redhat.com>
cc: Dave Chinner <david@fromorbit.com>, Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@elte.hu>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Anton Arapov <anton@redhat.com>, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 1/2] brw_mutex: big read-write mutex
In-Reply-To: <20121026141238.GA28443@redhat.com>
Message-ID: <Pine.LNX.4.64.1210261208010.5413@file.rdu.redhat.com>
References: <Pine.LNX.4.64.1210181525010.32376@file.rdu.redhat.com>
 <1350650286.30157.28.camel@twins> <Pine.LNX.4.64.1210191058060.13298@file.rdu.redhat.com>
 <1350668451.2768.60.camel@twins> <Pine.LNX.4.64.1210191758570.12753@file.rdu.redhat.com>
 <20121024030845.GT4291@dastard> <Pine.LNX.4.64.1210251008330.1849@file.rdu.redhat.com>
 <20121025234048.GH29378@dastard> <20121026120605.GA23545@redhat.com>
 <Pine.LNX.4.64.1210260900170.24474@file.rdu.redhat.com> <20121026141238.GA28443@redhat.com>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org



On Fri, 26 Oct 2012, Oleg Nesterov wrote:

> > The code is different, but it can be changed to use percpu rw semaphores
> > (if we add percpu_down_write_trylock).
> 
> I don't really understand how you can make percpu_down_write_trylock()
> atomic so that it can be called under br_write_lock(vfsmount_lock) in
> sb_prepare_remount_readonly(). So I guess you also need to replace
> vfsmount_lock at least. Or _trylock needs the barriers in _down_read.
> Or I missed something.
> 
> Oleg.

That's true - that code is under spinlock and you can't implement 
non-blocking percpu_down_write_trylock.

Mikulas