From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754754Ab2HTP5E (ORCPT ); Mon, 20 Aug 2012 11:57:04 -0400 Received: from merlin.infradead.org ([205.233.59.134]:43062 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752860Ab2HTP5C convert rfc822-to-8bit (ORCPT ); Mon, 20 Aug 2012 11:57:02 -0400 Message-ID: <1345478211.23018.69.camel@twins> Subject: Re: lockdep trace from posix timers From: Peter Zijlstra To: Oleg Nesterov Cc: Dave Jones , Linux Kernel , Thomas Gleixner , rostedt , dhowells , Al Viro Date: Mon, 20 Aug 2012 17:56:51 +0200 In-Reply-To: <20120820154154.GB20258@redhat.com> References: <20120724203613.GA9637@redhat.com> <1345140478.29668.54.camel@twins> <20120817151447.GA7918@redhat.com> <1345446957.23018.14.camel@twins> <1345463081.23018.34.camel@twins> <20120820150507.GC18499@redhat.com> <1345475530.23018.50.camel@twins> <20120820154154.GB20258@redhat.com> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.2- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2012-08-20 at 17:41 +0200, Oleg Nesterov wrote: > I won't insist. The patch I sent uses PF_EXITING and the fake > "struct callback_head* TWORK_EXITED", but this looks almost the same. Right, I used a fake callback_head because it avoided a few special cases since its a dereferencable pointer. > > > Note also your patch breaks fifo, but this is fixable. > > > > Why do you care about the order? > > IMHO, this is just more natural. Depends on what you're used to I guess ;-) Both RCU and irq_work are filo, this seems to be the natural way for single linked lists. > For example. keyctl_session_to_parent() does _cancel only to protect > from exploits doing keyctl(KEYCTL_SESSION_TO_PARENT) in an endless > loop. It could simply do task_work_add(), but in this case we need > fifo for correctness. I'm not entirely sure I see, not doing the cancel would delay the free until the executing of key_change_session_keyring()? doing that keyctl() in an indefinite loop involves going back to userspace, so where's the resource issue? Also, I'm not seeing where the FIFO requirement comes from. > > Iterating a single linked queue in fifo > > seems more expensive than useful. > > Currently the list is fifo (we add to the last element), this is O(1). depends on what way you look at the list I guess, with a single linked list there's only one end you can add to in O(1), so we're calling that the tail? > But the list should be short, we can reverse it in _run() if we change > task_work_add() to add to the head. Reversing a (single linked) list is O(n^2).. which is indeed doable for short lists, but why assume its short?