Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964892AbWIJUdy (ORCPT ); Sun, 10 Sep 2006 16:33:54 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S964893AbWIJUdy (ORCPT ); Sun, 10 Sep 2006 16:33:54 -0400 Received: from taganka54-host.corbina.net ([213.234.233.54]:1175 "EHLO mail.screens.ru") by vger.kernel.org with ESMTP id S964892AbWIJUdx (ORCPT ); Sun, 10 Sep 2006 16:33:53 -0400 Date: Mon, 11 Sep 2006 00:33:42 +0400 From: Oleg Nesterov To: "Eric W. Biederman" Cc: Andrew Morton , linux-kernel@vger.kernel.org, Linux Containers , Alan Cox Subject: Re: [PATCH] vt: Rework the console spawning variables. Message-ID: <20060910203324.GA121@oleg> References: <20060910142942.GA7384@oleg> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1673 Lines: 56 On 09/10, Eric W. Biederman wrote: > > Oleg Nesterov writes: > > > On 09/09, Eric W. Biederman wrote: > >> > >> This patch does several things. > >> - The variables used are moved into a structure and declared in vt_kern.h > >> - A spinlock is added so we don't have SMP races updating the values. > >> - Instead of raw pid_t value a struct_pid is used to guard against > >> pid wrap around issues, if the daemon to spawn a new console dies. > > > > I am not arguing against this patch, but it's a pity we can't use 'struct pid' > > lockless. What dou you think about this: > > Actually with xchg I can use a reference counted struct pid lockless. > > ... > > Perhaps: > void update_pid(struct pid **ref, struct pid *new) > { > struct pid *old; > get_pid(new); > old = xchg(ref, new); > put_pid(old); > } This can't work. This put_pid() can actually free the memory, while 'old' is still in use (lockless). > rcu is definitely not the solution in these cases as the struct pid > is stored for a long time so we need the reference count. Surely we need the reference count, I don't understand you. Look at put_pid_rcu(). That said, > In the general case you have more then one variable you want to keep > in sync and you need the lock for that. Yes. > But since I can write it as a moderately clear one liner in the > case that matters I don't much care. Ok. Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/