From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A3B2FC54FCB for ; Fri, 24 Apr 2020 17:39:58 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 06B03206B9 for ; Fri, 24 Apr 2020 17:39:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="Zm+H2R6y" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 06B03206B9 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-18633-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 13631 invoked by uid 550); 24 Apr 2020 17:39:51 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 13604 invoked from network); 24 Apr 2020 17:39:50 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587749978; bh=ykdr8Ad8jl8gHvtaRn/WVhFxDPP4Du64k/eazMGYOcg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Zm+H2R6y6P9TayzzrFFqaP9FIGHRcB5Zf9vvEDfKBAa6HRNHevNmJoa9cD2B46+de ZI+jYRehLuoBGmabynajkJb7OUlD6KpgjQ8k0bDc8XQDc0QaY7FzSXVdr0j918B5gG dtpAwTMmv2RPzj/xWWZd7COR04RQ5XSNv7tuatCo= Date: Fri, 24 Apr 2020 18:39:33 +0100 From: Will Deacon To: "Paul E. McKenney" Cc: Jann Horn , Peter Zijlstra , kernel list , Eric Dumazet , Kees Cook , Maddie Stone , Marco Elver , Thomas Gleixner , kernel-team , Kernel Hardening , Oleg Nesterov Subject: Re: [RFC PATCH 03/21] list: Annotate lockless list primitives with data_race() Message-ID: <20200424173932.GK21141@willie-the-truck> References: <20200324153643.15527-1-will@kernel.org> <20200324153643.15527-4-will@kernel.org> <20200324165128.GS20696@hirez.programming.kicks-ass.net> <20200324213200.GA21176@willie-the-truck> <20200330231315.GZ19865@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200330231315.GZ19865@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) On Mon, Mar 30, 2020 at 04:13:15PM -0700, Paul E. McKenney wrote: > On Tue, Mar 24, 2020 at 09:32:01PM +0000, Will Deacon wrote: > > [mutt crashed while I was sending this; apologies if you receive it twice] > > > > On Tue, Mar 24, 2020 at 05:56:15PM +0100, Jann Horn wrote: > > > On Tue, Mar 24, 2020 at 5:51 PM Peter Zijlstra wrote: > > > > On Tue, Mar 24, 2020 at 03:36:25PM +0000, Will Deacon wrote: > > > > > diff --git a/include/linux/list.h b/include/linux/list.h > > > > > index 4fed5a0f9b77..4d9f5f9ed1a8 100644 > > > > > --- a/include/linux/list.h > > > > > +++ b/include/linux/list.h > > > > > @@ -279,7 +279,7 @@ static inline int list_is_last(const struct list_head *list, > > > > > */ > > > > > static inline int list_empty(const struct list_head *head) > > > > > { > > > > > - return READ_ONCE(head->next) == head; > > > > > + return data_race(READ_ONCE(head->next) == head); > > > > > } > > > > > > > > list_empty() isn't lockless safe, that's what we have > > > > list_empty_careful() for. > > > > > > That thing looks like it could also use some READ_ONCE() sprinkled in... > > > > Crikey, how did I miss that? I need to spend some time understanding the > > ordering there. > > > > So it sounds like the KCSAN splats relating to list_empty() and loosely > > referred to by 1c97be677f72 ("list: Use WRITE_ONCE() when adding to lists > > and hlists") are indicative of real bugs and we should actually restore > > list_empty() to its former glory prior to 1658d35ead5d ("list: Use > > READ_ONCE() when testing for empty lists"). Alternatively, assuming > > list_empty_careful() does what it says on the tin, we could just make that > > the default. > > The list_empty_careful() function (suitably annotated) returns false if > the list is non-empty, including when it is in the process of becoming > either empty or non-empty. It would be fine for the lockless use cases > I have come across. Hmm, I had a look at the implementation and I'm not at all convinced that it's correct. First of all, the comment above it states: * NOTE: using list_empty_careful() without synchronization * can only be safe if the only activity that can happen * to the list entry is list_del_init(). Eg. it cannot be used * if another CPU could re-list_add() it. but it seems that people disregard this note and instead use it as a general-purpose lockless test, taking a lock and rechecking if it returns non-empty. It would also mean we'd have to keep the WRITE_ONCE() in INIT_LIST_HEAD, which is something that I've been trying to remove. In the face of something like a concurrent list_add(); list_add_tail() sequence, then the tearing writes to the head->{prev,next} pointers could cause list_empty_careful() to indicate that the list is momentarily empty. I've started looking at whether we can use a NULL next pointer to indicate an empty list, which might allow us to kill the __list_del_clearprev() hack at the same time, but I've not found enough time to really get my teeth into it yet. Will