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=-8.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,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 56CADC43331 for ; Tue, 31 Mar 2020 11:39:10 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 9E41720838 for ; Tue, 31 Mar 2020 11:39:09 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="dGAFrVwc" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9E41720838 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-18324-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 32574 invoked by uid 550); 31 Mar 2020 11:39:03 -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 3623 invoked from network); 30 Mar 2020 23:06:08 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585609556; bh=sZoG49IlE//Cd5eYwc8Z5AQTYaI/Iwc0/gOeae7abfU=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=dGAFrVwci1oqhpIsKl9e9+zoXRjlgxHw5cpzSTUVgiES3WLX3ScneiKQilOC6lGcZ XNawhAWA3v2G4nG3wtxPEVFjvFb1Mh1GTLuJLeJxcfZntUMVSTQJtiGPphMx3BDEpL ndPHqyKu/FUvPHQAgJPIuod7vfZ1BCr9hfKUTdjg= Date: Mon, 30 Mar 2020 16:05:55 -0700 From: "Paul E. McKenney" To: Will Deacon Cc: linux-kernel@vger.kernel.org, Eric Dumazet , Jann Horn , Kees Cook , Maddie Stone , Marco Elver , Peter Zijlstra , Thomas Gleixner , kernel-team@android.com, kernel-hardening@lists.openwall.com Subject: Re: [RFC PATCH 01/21] list: Remove hlist_unhashed_lockless() Message-ID: <20200330230555.GX19865@paulmck-ThinkPad-P72> References: <20200324153643.15527-1-will@kernel.org> <20200324153643.15527-2-will@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200324153643.15527-2-will@kernel.org> User-Agent: Mutt/1.9.4 (2018-02-28) On Tue, Mar 24, 2020 at 03:36:23PM +0000, Will Deacon wrote: > Commit c54a2744497d ("list: Add hlist_unhashed_lockless()") added a > "lockless" variant of hlist_unhashed() that uses READ_ONCE() to access > the 'pprev' pointer of the 'hlist_node', the intention being that this > could be used by 'timer_pending()' to silence a KCSAN warning. As well > as forgetting to add the caller, the patch also sprinkles > {READ,WRITE}_ONCE() invocations over the standard (i.e. non-RCU) hlist > code, which is undesirable for a number of reasons: > > 1. It gives the misleading impression that the non-RCU hlist code is > in some way lock-free (despite the notable absence of any memory > barriers!) and silences KCSAN in such cases. > > 2. It unnecessarily penalises code generation for non-RCU hlist users > > 3. It makes it difficult to introduce list integrity checks because > of the possibility of concurrent callers. > > Retain the {READ,WRITE}_ONCE() invocations for the RCU hlist code, but > remove them from the non-RCU implementation. Remove the unused > 'hlist_unhashed_lockless()' function entirely and add the READ_ONCE() > to hlist_unhashed(), as we do already for hlist_empty() already. > > Cc: Eric Dumazet > Cc: Thomas Gleixner > Cc: Paul E. McKenney > Cc: Peter Zijlstra > Signed-off-by: Will Deacon No objection, however 90c018942c2b ("timer: Use hlist_unhashed_lockless() in timer_pending()") in -tip must change from hlist_unhashed_lockless() to hlist_unhashed(). Easy fix, though, so: Acked-by: Paul E. McKenney > --- > include/linux/list.h | 33 ++++++++++----------------------- > 1 file changed, 10 insertions(+), 23 deletions(-) > > diff --git a/include/linux/list.h b/include/linux/list.h > index 884216db3246..4fed5a0f9b77 100644 > --- a/include/linux/list.h > +++ b/include/linux/list.h > @@ -777,19 +777,6 @@ static inline void INIT_HLIST_NODE(struct hlist_node *h) > * node in unhashed state, but hlist_nulls_del() does not. > */ > static inline int hlist_unhashed(const struct hlist_node *h) > -{ > - return !h->pprev; > -} > - > -/** > - * hlist_unhashed_lockless - Version of hlist_unhashed for lockless use > - * @h: Node to be checked > - * > - * This variant of hlist_unhashed() must be used in lockless contexts > - * to avoid potential load-tearing. The READ_ONCE() is paired with the > - * various WRITE_ONCE() in hlist helpers that are defined below. > - */ > -static inline int hlist_unhashed_lockless(const struct hlist_node *h) > { > return !READ_ONCE(h->pprev); > } > @@ -852,11 +839,11 @@ static inline void hlist_del_init(struct hlist_node *n) > static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h) > { > struct hlist_node *first = h->first; > - WRITE_ONCE(n->next, first); > + n->next = first; > if (first) > - WRITE_ONCE(first->pprev, &n->next); > + first->pprev = &n->next; > WRITE_ONCE(h->first, n); > - WRITE_ONCE(n->pprev, &h->first); > + n->pprev = &h->first; > } > > /** > @@ -867,9 +854,9 @@ static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h) > static inline void hlist_add_before(struct hlist_node *n, > struct hlist_node *next) > { > - WRITE_ONCE(n->pprev, next->pprev); > - WRITE_ONCE(n->next, next); > - WRITE_ONCE(next->pprev, &n->next); > + n->pprev = next->pprev; > + n->next = next; > + next->pprev = &n->next; > WRITE_ONCE(*(n->pprev), n); > } > > @@ -881,12 +868,12 @@ static inline void hlist_add_before(struct hlist_node *n, > static inline void hlist_add_behind(struct hlist_node *n, > struct hlist_node *prev) > { > - WRITE_ONCE(n->next, prev->next); > - WRITE_ONCE(prev->next, n); > - WRITE_ONCE(n->pprev, &prev->next); > + n->next = prev->next; > + prev->next = n; > + n->pprev = &prev->next; > > if (n->next) > - WRITE_ONCE(n->next->pprev, &n->next); > + n->next->pprev = &n->next; > } > > /** > -- > 2.20.1 >