Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756497Ab2BWO3f (ORCPT ); Thu, 23 Feb 2012 09:29:35 -0500 Received: from mail-lpp01m010-f46.google.com ([209.85.215.46]:39284 "EHLO mail-lpp01m010-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756185Ab2BWO3d convert rfc822-to-8bit (ORCPT ); Thu, 23 Feb 2012 09:29:33 -0500 Authentication-Results: mr.google.com; spf=pass (google.com: domain of jiangtao.jit@gmail.com designates 10.152.104.143 as permitted sender) smtp.mail=jiangtao.jit@gmail.com; dkim=pass header.i=jiangtao.jit@gmail.com MIME-Version: 1.0 In-Reply-To: References: <4F411373.50304@gmail.com> Date: Thu, 23 Feb 2012 22:29:31 +0800 Message-ID: Subject: Re: A problem with percpu variable cpu_number From: Tao Jiang To: Hillf Danton Cc: Brian Gerst , Cong Wang , linux-kernel@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2743 Lines: 74 Hillf Danton: Thanks for your remind and sorry for my poor English. 2012/2/22 Hillf Danton : > On Wed, Feb 22, 2012 at 7:13 PM, Tao Jiang wrote: >> Hi: >> >> Thank you all. >> >> So in boot_cpu_init(), it will always set bit 0 to these masks. >> If the boot cpu is the first processor, it's the right case. >> And if the BP is not the first one, is it wrong? >> But can it happen that the BP is not cpu0? > > BP could be not "the first one". > Due to that current code boots fine, I guess, you mess > logic CPU with hard CPU. > > btw, replying messages in this way looks not fine. > >> Thank you. >> >> >> 2012/2/22 Brian Gerst : >>> On Tue, Feb 21, 2012 at 9:29 AM, Cong Wang wrote: >>>> On 02/21/2012 08:41 PM, Tao Jiang wrote: >>>>> >>>>> Hi Cong Wang: >>>>> >>>>> I read the file vmlinux.lds.S in arch/x86/kernel >>>>> section .data..percpu is between .init.data and .init.end >>>>> Is that means these percpu variables will be freed after init? >>>> >>>> >>>> % grep -e __init_begin -e __init_end -e __per_cpu_start -e __per_cpu_end >>>> /boot/System.map >>>> 0000000000000000 D __per_cpu_start >>>> 0000000000014bc0 D __per_cpu_end >>>> ffffffff81cf3000 D __init_begin >>>> ffffffff81dfc000 R __init_end >>>> % objdump -d -j .data..percpu vmlinux | grep cpu_number >>>> 000000000000dc38 : >>> >>> The .data..percpu section is placed in the init section, but x86-64 is >>> a special case as noted below. ?The boot cpu is pointed to the init >>> percpu section until setup_per_cpu_areas() is called, when it switches >>> to the regular percpu area. ?The init percpu data is then freed with >>> all other init data. >>> >>> The reason the percpu symbols start at virtual address 0 on x86-64 is >>> because of the requirement that gs_base must be a canonical address >>> (it cannot be a simple offset like x86-32). ?But, the data is still >>> loaded in the init section in memory. ?See >>> arch/x86/kernel/vmlinux.lds.S for the explanation of how the linker >>> changes the program headers to set the virtual address to zero but >>> keeps the load address in the init section. >>> >>> To answer the original question. in the case of cpu_number, it is set >>> to zero in the init section because it doesn't have an explicit >>> initializer. ?Therefore the boot cpu will always read zero for the >>> cpu_number, even before setup_per_cpu_areas() is called. >>> >>> >>> -- >>> Brian Gerst >> -- -- 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/