From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757879Ab2DIWIu (ORCPT ); Mon, 9 Apr 2012 18:08:50 -0400 Received: from mta114.f1.k8.com.br ([187.73.32.186]:54937 "EHLO mta114.f1.k8.com.br" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751565Ab2DIWIt (ORCPT ); Mon, 9 Apr 2012 18:08:49 -0400 Message-ID: <1334009322.2444.12.camel@Thor> Subject: Re: [PATCH 1/1] rculist: Made list_first_entry_rcu usable From: Michel Machado To: paulmck@linux.vnet.ibm.com Cc: Dipankar Sarma , linux-kernel@vger.kernel.org Date: Mon, 09 Apr 2012 18:08:42 -0400 In-Reply-To: <20120409212440.GL2430@linux.vnet.ibm.com> References: <1333417354.2412.7.camel@Thor> <20120409212440.GL2430@linux.vnet.ibm.com> Organization: Digirati Internet Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.2- Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2012-04-09 at 14:24 -0700, Paul E. McKenney wrote: > On Mon, Apr 02, 2012 at 09:42:34PM -0400, Michel Machado wrote: > > The macro list_first_entry_rcu assumed that the passed list is not empty > > as its counterpart list_first_entry does. However, one can test that a > > list is not empty with list_empty before calling list_first_entry, > > whereas neither exists list_empty_rcu, nor is advisable to add it as the > > example below shows. > > > > Assuming that list_empty_rcu is available, one could write the following > > snippet: > > > > if (!list_empty_rcu(mylist)) { > > struct foo *bar = list_first_entry_rcu(mylist, struct foo, > > list_member); > > do_something(bar); > > } > > > > The problem with this snippet is the following racing condition: the > > list may not be empty when list_empty_rcu checks it, but it may be when > > list_first_entry_rcu rereads the ->next pointer. > > > > This patch cannot break any upstream code because list_first_entry_rcu > > is not being used anywhere in the kernel (tested with grep(1)), and > > external code that uses it is probably broken already. > > Hello, Michel, > > Interesting point! > > Are you intending to use list_first_entry_rcu()? If not, perhaps the > best thing to do is to remove it. > > Thanx, Paul Hi Paul, I'd rather keep list_first_entry_rcu(). I've already used it twice in the project I'm working on (https://github.com/AltraMayor/XIA-for-Linux), and I expect to submit this work upstream once it reaches reasonable quality as you can check in the roadmap available here: https://github.com/AltraMayor/XIA-for-Linux/wiki/Roadmap#wiki-Making_into_Linus_source_tree Not to mention that, given the subtlety of the problem, removing list_first_entry_rcu() may introduce the same bug whenever someone tries to mimic list_first_entry(), and having it in the kernel helps to guide those with an example. [ ]'s Michel Machado > > > Signed-off-by: Michel Machado > > CC: Dipankar Sarma > > CC: "Paul E. McKenney" > > --- > > Please CC my e-mail address while replying this message because I don't > > subscribe this mailing list due to its high volume; thanks. > > > > diff --git a/include/linux/rculist.h b/include/linux/rculist.h > > index d079290..866d3ec 100644 > > --- a/include/linux/rculist.h > > +++ b/include/linux/rculist.h > > @@ -233,13 +233,16 @@ static inline void list_splice_init_rcu(struct > > list_head *list, > > * @type: the type of the struct this is embedded in. > > * @member: the name of the list_struct within the struct. > > * > > - * Note, that list is expected to be not empty. > > + * Note that if the list is empty, it returns NULL. > > * > > * This primitive may safely run concurrently with the _rcu > > list-mutation > > * primitives such as list_add_rcu() as long as it's guarded by > > rcu_read_lock(). > > */ > > #define list_first_entry_rcu(ptr, type, member) \ > > - list_entry_rcu((ptr)->next, type, member) > > + ({struct list_head *__ptr = ptr; \ > > + struct list_head __rcu *__next = list_next_rcu(__ptr); \ > > + likely(__ptr != __next) ? container_of(__next, type, member) : NULL; > > \ > > + }) > > > > /** > > * list_for_each_entry_rcu - iterate over rcu list of given type > > > > >