From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933535AbcFQPAF (ORCPT ); Fri, 17 Jun 2016 11:00:05 -0400 Received: from merlin.infradead.org ([205.233.59.134]:40597 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932065AbcFQPAD (ORCPT ); Fri, 17 Jun 2016 11:00:03 -0400 Date: Fri, 17 Jun 2016 16:59:54 +0200 From: Peter Zijlstra To: Oleg Drokin Cc: Mailing List , mingo@redhat.com, arjan@linux.intel.com, "J . Bruce Fields" , Jeff Layton Subject: Re: initialize a mutex into locked state? Message-ID: <20160617145954.GW30154@twins.programming.kicks-ass.net> References: <55315ADD-4AE1-4EA4-8291-1B07F659BD99@linuxhacker.ru> <20160617082559.GD30935@twins.programming.kicks-ass.net> <38111D58-C42F-4ED3-BC31-04884795BE38@linuxhacker.ru> <20160617141912.GS30154@twins.programming.kicks-ass.net> <20160617143200.GU30154@twins.programming.kicks-ass.net> <17E58758-CA23-420D-89B4-50E71F8A6428@linuxhacker.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <17E58758-CA23-420D-89B4-50E71F8A6428@linuxhacker.ru> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 17, 2016 at 10:40:14AM -0400, Oleg Drokin wrote: > >> The problem is: > >> > >> spin_lock(somelock); > >> structure = some_internal_list_lookup(list); > >> if (structure) > >> goto out; > >> > >> init_new_structure(new_structure); > >> mutex_init(&new_structure->s_mutex); > >> mutex_lock(&new_structure->s_mutex); // XXX CANNOT DO THIS UNDER SPINLOCK! > > > > mutex_trylock(&new_structure->s_mutex); > > > > should work, since you know it cannot be acquired yet by anybody else, > > since you've not published it yet. > > This does work, but suddenly does not look so obvious anymore, does it? Well, the whole thing is somewhat tricky and would require a comment anyway. > I got some feedback that doing this is not really preferred. Much preferred to adding additional API I would think. Because if we add it here, we'll also have to add it to rt_mutex. Also, I'm not sure I want to promote this construct, it seems like a very special case. > Also once __must_check is added to mutex_try_lock() (surprised it's not yet), > we'll need to also have the useless "but what if it did fail to lock" path? The BUG_ON() suggested by Jeff seems like a good solution ;-)