From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753778Ab3AQWqH (ORCPT ); Thu, 17 Jan 2013 17:46:07 -0500 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.122]:19120 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751389Ab3AQWqF (ORCPT ); Thu, 17 Jan 2013 17:46:05 -0500 X-Authority-Analysis: v=2.0 cv=O9a7TWBW c=1 sm=0 a=rXTBtCOcEpjy1lPqhTCpEQ==:17 a=mNMOxpOpBa8A:10 a=1dK1Tq4abSwA:10 a=5SG0PmZfjMsA:10 a=Q9fys5e9bTEA:10 a=meVymXHHAAAA:8 a=daFlbPKOXuoA:10 a=ZE2Y4y6o2NVEFqsLjbsA:9 a=PUjeQqilurYA:10 a=rXTBtCOcEpjy1lPqhTCpEQ==:117 X-Cloudmark-Score: 0 X-Authenticated-User: X-Originating-IP: 74.67.115.198 Message-ID: <1358462763.23211.57.camel@gandalf.local.home> Subject: Re: [RFC][PATCH] slub: Check for page NULL before doing the node_match check From: Steven Rostedt To: Christoph Lameter Cc: LKML , linux-mm , Andrew Morton , Pekka Enberg , Matt Mackall , Thomas Gleixner , RT , Clark Williams , John Kacur , "Luis Claudio R. Goncalves" Date: Thu, 17 Jan 2013 17:46:03 -0500 In-Reply-To: <0000013c4a7e7fbf-c51fd42a-2455-4fec-bb37-915035956f05-000000@email.amazonses.com> References: <1358446258.23211.32.camel@gandalf.local.home> <1358447864.23211.34.camel@gandalf.local.home> <0000013c4a69a2cf-1a19a6f6-e6a3-4f06-99a4-10fdd4b9aca2-000000@email.amazonses.com> <1358458996.23211.46.camel@gandalf.local.home> <0000013c4a7e7fbf-c51fd42a-2455-4fec-bb37-915035956f05-000000@email.amazonses.com> Content-Type: text/plain; charset="ISO-8859-15" X-Mailer: Evolution 3.4.4-1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2013-01-17 at 21:51 +0000, Christoph Lameter wrote: > This is dealing with the same cpu being interrupted. Some of these > segments are in interrupt disable sections so they are not affected. Except that we are not always on the same CPU. Now I'm looking at mainline (non modified by -rt): >>From slab_alloc_node(): /* * Must read kmem_cache cpu data via this cpu ptr. Preemption is * enabled. We may switch back and forth between cpus while * reading from one cpu area. That does not matter as long * as we end up on the original cpu again when doing the cmpxchg. */ c = __this_cpu_ptr(s->cpu_slab); /* * The transaction ids are globally unique per cpu and per operation on * a per cpu queue. Thus they can be guarantee that the cmpxchg_double * occurs on the right processor and that there was no operation on the * linked list in between. */ tid = c->tid; barrier(); object = c->freelist; page = c->page; if (unlikely(!object || !node_match(page, node))) object = __slab_alloc(s, gfpflags, node, addr, c); Where we hit the bug on -rt, and can most certainly do it on mainline. This code does not disable preemption (the comment even states that). So if we switch CPUs after reading __this_cpu_ptr(), we are still accessing the 'c' pointer of the CPU we left. Hence, there's nothing protecting c->page being NULL when c->freelist is not NULL. -- Steve