From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752341Ab0C0HCV (ORCPT ); Sat, 27 Mar 2010 03:02:21 -0400 Received: from ksp.mff.cuni.cz ([195.113.26.206]:53820 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751909Ab0C0HCU (ORCPT ); Sat, 27 Mar 2010 03:02:20 -0400 Date: Sat, 27 Mar 2010 08:02:07 +0100 From: Pavel Machek To: Jiri Slaby Cc: "Rafael J. Wysocki" , linux-pm@lists.linux-foundation.org, linux-kernel@vger.kernel.org, Nigel Cunningham Subject: Re: [RFC 09/15] PM / Hibernate: user, implement user_ops writer Message-ID: <20100327070207.GQ18689@elf.ucw.cz> References: <1269361063-3341-1-git-send-email-jslaby@suse.cz> <20100324204259.GA6423@elf.ucw.cz> <4BAA86E8.5090108@gmail.com> <201003252314.33256.rjw@sisk.pl> <4BAC7FC3.50405@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4BAC7FC3.50405@gmail.com> X-Warning: Reading this can be dangerous to your mental health. User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 2010-03-26 10:34:59, Jiri Slaby wrote: > On 03/25/2010 11:14 PM, Rafael J. Wysocki wrote: > > On Wednesday 24 March 2010, Jiri Slaby wrote: > >> On 03/24/2010 09:42 PM, Pavel Machek wrote: > >>>> + if (test_bit(TODO_CLOSED, to_do_flags)) > >>>> + return -EIO; > >>>> + > >>>> + to_do_buf = buf; > >>>> + wmb(); > >>>> + set_bit(TODO_WORK, to_do_flags); > >>>> + wake_up_interruptible(&to_do_wait); > >>> > >>> Uhuh, open-coded barriers... these need to be commented, and I guess > >>> you just should not play this kind of trickery. > >> > >> It's just to ensure the to_do_buf store is not reordered with the > >> set_bit. I wanted to avoid locks as too heavy tools here. > > > > No, please use them, at least in a prototype version. > > > > We can always optimize things out later, but doing optimizations upfront > > doesn't really work well from my experience. > > > > So, if you'd use a lock somewhere, please use it, or maybe use a completion if > > that fits the design better. > > That's it, I don't think a lock is appropriate here (I didn't even think > of that) -- I don't know what to lock (OK, I see it, but it's not that > clear). There is no potential for race per se, I only need to disable > reordering (which locks do as a side-effect). I need the steps to be > done in the A-B order where there is a barrier appropriate. Here, A is > store to to_do_buf, B is set_bit. It's I set to_do_buf, flag that it may > be used, the consumer will see the flag and use to_do_buf, in this > order. Could you just use same locking user.c uses? It does not really do any magic... Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html