linux-kernel - Re: A problem with percpu variable cpu

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Date:	Wed, 22 Feb 2012 21:21:06 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Tao Jiang <jiangtao.jit@...il.com>
Cc:	Brian Gerst <brgerst@...il.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: A problem with percpu variable cpu_number

On Wed, Feb 22, 2012 at 7:13 PM, Tao Jiang <jiangtao.jit@...il.com> 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 <brgerst@...il.com>:
>> On Tue, Feb 21, 2012 at 9:29 AM, Cong Wang <xiyou.wangcong@...il.com> 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 <cpu_number>:
>>
>> 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
> --
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