From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=dqcu=6X=vger.kernel.org=linux-kernel-owner@kernel.org>
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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED,
	DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,
	SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham
	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 DA5F1C28CBC
	for <linux-kernel@archiver.kernel.org>; Sat,  9 May 2020 04:33:11 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [23.128.96.18])
	by mail.kernel.org (Postfix) with ESMTP id A6B9F2184D
	for <linux-kernel@archiver.kernel.org>; Sat,  9 May 2020 04:33:11 +0000 (UTC)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org;
	s=default; t=1588998791;
	bh=O04g/ed99TB3Q1nZ1fxfTeIntZZXj5bXO9bE+5e5nn0=;
	h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID:
	 From;
	b=JUIoemKnGfZlChJtuZ0TSzdHrO7Qrj7Fgu/G7SHNmT43sCx8zGxcQkSX2MvRzB9Lt
	 /SIj51ht36pDh450DXLtU0HYZuG+fYj2Jd2Gx9YbJyLlmF3GImzi9D4i6ZIuOMPw0w
	 rw/kF+tSNAQu/BsglCIamqd+L9M7tPOc4/P2EwIw=
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1726013AbgEIEdJ (ORCPT
        <rfc822;linux-kernel@archiver.kernel.org>);
        Sat, 9 May 2020 00:33:09 -0400
Received: from mail.kernel.org ([198.145.29.99]:39070 "EHLO mail.kernel.org"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1725795AbgEIEdI (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Sat, 9 May 2020 00:33:08 -0400
Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78])
        (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
        (No client certificate requested)
        by mail.kernel.org (Postfix) with ESMTPSA id 5291020735;
        Sat,  9 May 2020 04:33:08 +0000 (UTC)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org;
        s=default; t=1588998788;
        bh=O04g/ed99TB3Q1nZ1fxfTeIntZZXj5bXO9bE+5e5nn0=;
        h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From;
        b=F5+NMPaGYzuEyMVkGH22M/WOTFbi8m7oVsGmcdRL8rq9bixg1EYlEUOHuAWVMBws1
         6vSdb+hshPWWKUh+T6nIF9o8KrmFZ1PvZicYk/HjLK/wIZjbSpf6QfImT8ALngqfEF
         gymPJLZBSF0gMYH/2bKmLy3O6SBTkcY6zp1Xkb6g=
Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000)
        id 34A773523058; Fri,  8 May 2020 21:33:08 -0700 (PDT)
Date:   Fri, 8 May 2020 21:33:08 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     Will Deacon <will@kernel.org>, Elver Marco <elver@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: Re: [PATCH -next v2] locking/osq_lock: annotate a data race in
 osq_lock
Message-ID: <20200509043308.GL2869@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
References: <1581429255-12542-1-git-send-email-cai@lca.pw>
 <2C8BF141-5384-475F-B713-6D473557C65C@lca.pw>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <2C8BF141-5384-475F-B713-6D473557C65C@lca.pw>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, May 08, 2020 at 04:59:05PM -0400, Qian Cai wrote:
> 
> 
> > On Feb 11, 2020, at 8:54 AM, Qian Cai <cai@lca.pw> wrote:
> > 
> > prev->next could be accessed concurrently as noticed by KCSAN,
> > 
> > write (marked) to 0xffff9d3370dbbe40 of 8 bytes by task 3294 on cpu 107:
> >  osq_lock+0x25f/0x350
> >  osq_wait_next at kernel/locking/osq_lock.c:79
> >  (inlined by) osq_lock at kernel/locking/osq_lock.c:185
> >  rwsem_optimistic_spin
> >  <snip>
> > 
> > read to 0xffff9d3370dbbe40 of 8 bytes by task 3398 on cpu 100:
> >  osq_lock+0x196/0x350
> >  osq_lock at kernel/locking/osq_lock.c:157
> >  rwsem_optimistic_spin
> >  <snip>
> > 
> > Since the write only stores NULL to prev->next and the read tests if
> > prev->next equals to this_cpu_ptr(&osq_node). Even if the value is
> > shattered, the code is still working correctly. Thus, mark it as an
> > intentional data race using the data_race() macro.
> > 
> > Signed-off-by: Qian Cai <cai@lca.pw>
> 
> Hmm, this patch has been dropped from linux-next from some reasons.
> 
> Paul, can you pick this up along with KCSAN fixes?
> 
> https://lore.kernel.org/lkml/1581429255-12542-1-git-send-email-cai@lca.pw/

I have queued it on -rcu, but it is too late for v5.8 via the -rcu tree,
so this is v5.9 at the earliest.  Plus I would need an ack from one of
the locking folks.

							Thanx, Paul

> > ---
> > 
> > v2: insert some code comments.
> > 
> > kernel/locking/osq_lock.c | 6 +++++-
> > 1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > index 1f7734949ac8..f733bcd99e8a 100644
> > --- a/kernel/locking/osq_lock.c
> > +++ b/kernel/locking/osq_lock.c
> > @@ -154,7 +154,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> > 	 */
> > 
> > 	for (;;) {
> > -		if (prev->next == node &&
> > +		/*
> > +		 * cpu_relax() below implies a compiler barrier which would
> > +		 * prevent this comparison being optimized away.
> > +		 */
> > +		if (data_race(prev->next == node) &&
> > 		    cmpxchg(&prev->next, node, NULL) == node)
> > 			break;
> > 
> > -- 
> > 1.8.3.1
> > 
>