Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753998AbaJUFom (ORCPT ); Tue, 21 Oct 2014 01:44:42 -0400 Received: from mail-la0-f43.google.com ([209.85.215.43]:51277 "EHLO mail-la0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750831AbaJUFok (ORCPT ); Tue, 21 Oct 2014 01:44:40 -0400 MIME-Version: 1.0 In-Reply-To: <20141021054118.GA4420@pd.tnic> References: <93ffc13d9e829f3ded9776fec62385bccb7439e1.1413323612.git.luto@amacapital.net> <20141016154914.GD30314@nazgul.tnic> <20141021054118.GA4420@pd.tnic> From: Andy Lutomirski Date: Mon, 20 Oct 2014 22:44:18 -0700 Message-ID: Subject: Re: [RFC 3/5] x86: Add a comment clarifying LDT context switching To: Borislav Petkov Cc: Peter Zijlstra , Valdis Kletnieks , "linux-kernel@vger.kernel.org" , Paul Mackerras , Arnaldo Carvalho de Melo , Ingo Molnar , Kees Cook , Andrea Arcangeli , Erik Bosman Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 20, 2014 at 10:41 PM, Borislav Petkov wrote: > On Thu, Oct 16, 2014 at 09:21:42AM -0700, Andy Lutomirski wrote: >> I think it's the same as in the other case in switch_mm. leave_mm does >> cpumask_clear_cpu(cpu, mm_cpumask(active_mm)), and, once that has >> happened, modify_ldt won't send an IPI to this CPU. So, if leave_mm >> runs, and then another CPU calls modify_ldt on the mm that is in lazy >> mode here, it won't update our LDT register, so the LDT register and >> prev->context.ldt might not match. > > Ok, let me see if I can follow with an example: > > We call leave_mm() on, say, cpu 3 and mm_cpumask(active_mm) has cpu 3 and > 4 set. Then, on cpu 4 we call modify_ldt on that same mm and there in > alloc_ldt() we have this: > > if (!cpumask_equal(mm_cpumask(current->mm), > cpumask_of(smp_processor_id()))) > smp_call_function(flush_ldt, current->mm, 1); > > and since we've cleared cpu 3 from the cpumask, we don't flush_ldt() > on it and there you have the difference. > > Am I close? You're exactly correct, or at least you seem to understand it the way I do :) --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/