From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753380AbcERSB5 (ORCPT <rfc822;w@1wt.eu>);
	Wed, 18 May 2016 14:01:57 -0400
Received: from e34.co.us.ibm.com ([32.97.110.152]:52396 "EHLO
	e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751967AbcERSBz (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 18 May 2016 14:01:55 -0400
X-IBM-Helo: d03dlp03.boulder.ibm.com
X-IBM-MailFrom: paulmck@linux.vnet.ibm.com
X-IBM-RcptTo: linux-kernel@vger.kernel.org
Date: Wed, 18 May 2016 11:01:53 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>, dennis.chen@arm.com,
        jiangshanlai@gmail.com, josh@joshtriplett.org,
        LKML <linux-kernel@vger.kernel.org>, mathieu.desnoyers@efficios.com,
        Steven Rostedt <rostedt@goodmis.org>, steve.capper@arm.com,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH] rcu: tree: correctly handle sparse possible CPUs
Message-ID: <20160518180153.GA3528@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <1463417306-3826-1-git-send-email-mark.rutland@arm.com>
 <CAPAsAGzYxfEsc8EMVHr0NFP1x1CjtdHbi_GApfbu18of98mBvw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAPAsAGzYxfEsc8EMVHr0NFP1x1CjtdHbi_GApfbu18of98mBvw@mail.gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
X-TM-AS-MML: disable
X-Content-Scanned: Fidelis XPS MAILER
x-cbid: 16051818-0017-0000-0000-00002F433E90
X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, May 18, 2016 at 06:15:23PM +0300, Andrey Ryabinin wrote:
> 2016-05-16 19:48 GMT+03:00 Mark Rutland <mark.rutland@arm.com>:
> 
> >  /*
> > + * Iterate over all possible CPUs in a leaf RCU node.
> > + */
> > +#define for_each_leaf_node_possible_cpu(rnp, cpu) \
> > +       for ((cpu) = rnp->grplo; \
> > +            cpu <= rnp->grphi; \
> > +            cpu = cpumask_next((cpu), cpu_possible_mask))
> > +
> > +/*
> > + * Iterate over all possible CPUs in a leaf RCU node, at each step providing a
> > + * bit for comparison against rcu_node bitmasks.
> > + */
> > +#define for_each_leaf_node_possible_cpu_bit(rnp, cpu, bit) \
> > +       for ((cpu) = rnp->grplo, (bit) = 1; \
> > +            cpu <= rnp->grphi; \
> > +            cpu = cpumask_next((cpu), cpu_possible_mask), \
> > +                  (bit) = 1UL << (cpu - rnp->grplo))
> > +
> 
> [    0.163652] UBSAN: Undefined behaviour in ../kernel/rcu/tree.c:2912:3
> [    0.164000] shift exponent 64 is too large for 64-bit type 'long
> unsigned int'

Ah, dead value, but can happen nevertheless.  One fix is to prevent the
assignment to bit when cpu > rnp->grphi.

Any ideas for a better fix?  And isn't there some combination of
signedness that makes shifting all the bits out of the value defined
to zero?  Or is that only for right shifts?

							Thanx, Paul