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 S1756855AbbCMQ33 (ORCPT <rfc822;w@1wt.eu>);
	Fri, 13 Mar 2015 12:29:29 -0400
Received: from resqmta-ch2-06v.sys.comcast.net ([69.252.207.38]:48086 "EHLO
	resqmta-ch2-06v.sys.comcast.net" rhost-flags-OK-OK-OK-OK)
	by vger.kernel.org with ESMTP id S1755965AbbCMQ3Z (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 13 Mar 2015 12:29:25 -0400
Date: Fri, 13 Mar 2015 11:29:23 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
X-X-Sender: cl@gentwo.org
To: Mark Rutland <mark.rutland@arm.com>
cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        David Rientjes <rientjes@google.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Pekka Enberg <penberg@kernel.org>,
        Steve Capper <steve.capper@linaro.org>
Subject: Re: [PATCH] mm/slub: fix lockups on PREEMPT && !SMP kernels
In-Reply-To: <1426261632-8911-1-git-send-email-mark.rutland@arm.com>
Message-ID: <alpine.DEB.2.11.1503131126550.13899@gentwo.org>
References: <1426261632-8911-1-git-send-email-mark.rutland@arm.com>
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, 13 Mar 2015, Mark Rutland wrote:

>  	 */
> -	do {
> -		tid = this_cpu_read(s->cpu_slab->tid);
> -		c = raw_cpu_ptr(s->cpu_slab);
> -	} while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
> +	c = raw_cpu_ptr(s->cpu_slab);
> +	tid = READ_ONCE(c->tid);
>

Ok that works for the !SMP case. What about SMP and PREEMPT now?

And yes code like this was deemed safe for years and the race condition is
very subtle and difficult to trigger (also given that PREEMPT is rarely
used these days).