From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx183.postini.com [74.125.245.183]) by kanga.kvack.org (Postfix) with SMTP id 54FF46B0006 for ; Thu, 17 Jan 2013 17:46:05 -0500 (EST) 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 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" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: LKML , linux-mm , Andrew Morton , Pekka Enberg , Matt Mackall , Thomas Gleixner , RT , Clark Williams , John Kacur , "Luis Claudio R. Goncalves" 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org