From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752211Ab3ARSwO (ORCPT ); Fri, 18 Jan 2013 13:52:14 -0500 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.122]:6644 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751572Ab3ARSwN (ORCPT ); Fri, 18 Jan 2013 13:52:13 -0500 X-Authority-Analysis: v=2.0 cv=W/m6pGqk c=1 sm=0 a=rXTBtCOcEpjy1lPqhTCpEQ==:17 a=mNMOxpOpBa8A:10 a=HJMuRcDKq34A:10 a=5SG0PmZfjMsA:10 a=Q9fys5e9bTEA:10 a=meVymXHHAAAA:8 a=RPfT3mj6-UMA:10 a=bJxBL_JUdif7w-WkzrQA:9 a=PUjeQqilurYA:10 a=rXTBtCOcEpjy1lPqhTCpEQ==:117 X-Cloudmark-Score: 0 X-Authenticated-User: X-Originating-IP: 74.67.115.198 Message-ID: <1358535129.7383.25.camel@gandalf.local.home> Subject: Re: [RFC][PATCH v2] slub: Keep page and object in sync in slab_alloc_node() 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: Fri, 18 Jan 2013 13:52:09 -0500 In-Reply-To: <0000013c4eec1cbd-fb392b9f-b39e-4bcf-a043-2fa76fb8d35a-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> <1358462763.23211.57.camel@gandalf.local.home> <1358464245.23211.62.camel@gandalf.local.home> <1358464837.23211.66.camel@gandalf.local.home> <1358468598.23211.67.camel@gandalf.local.home> <1358468924.23211.69.camel@gandalf.local.home> <0000013c4e1ea131-b8ab56b9-bfca-44fe-b5da-f030551194c9-000000@email.amazonses.com> <1358521484.7383.8.camel@gandalf.local.home> <1358524501.7383.17.camel@gandalf.local.home> <0000013c4eec1cbd-fb392b9f-b39e-4bcf-a043-2fa76fb8d35a-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 Fri, 2013-01-18 at 18:29 +0000, Christoph Lameter wrote: > On Fri, 18 Jan 2013, Steven Rostedt wrote: > > > On Fri, 2013-01-18 at 10:04 -0500, Steven Rostedt wrote: > > > > Just to be more complete: > > > > > CPU0 CPU1 > > > ---- ---- > > c = __this_cpu_ptr(s->cpu_slab); > > > > > > > page> > > > > > > > updates c->tid > > We can avoid the above scenario by doing a cpu local fetch. > > i.e. > ` tid = this_cpu_read(s->cpu_slab->tid); I'm curious to why not just add the preempt disable? It's rather quick and avoids all this complex trickery, which is just prone to bugs. It would make it much easier for others to review as well, and also keeps the setting of page, objects and cpu_slab consistent with everything else (which is assigned under preempt(irq)_disable). > > > > > updates c->page > > > updates c->freelist > > > tid> > > > freelist> > > > > > > node_match() succeeds even though > > > current c->page wont > > > > > > > > > > > > this_cpu_cmpxchg_double() only tests > > > the object (freelist) and tid, both which > > > will match, but the page that was tested > > > isn't the right one. > > > > > > > Yes, it's very unlikely, but we are in the business of dealing with the > > very unlikely. That's because in our business, the very unlikely is very > > likely. Damn, I need to buy a lotto ticket! > > Well, the consequence would be that an object from another node than > desired will be allocated. Not that severe of an issue. Yes, it's not that severe of an issue, but it is still incorrect code. Why not just allocate on whatever node you want then? Why bother with the check at all? -- Steve