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Date:   Mon, 16 Jan 2017 10:36:14 -0800
From:   Stephane Eranian <>
To:     zhouchengming <>
Cc:     Thomas Gleixner <>,
        LKML <>, x86 <>,
        "" <>,
        Peter Zijlstra <>,
        "H. Peter Anvin" <>,
        "" <>,
        "Liang, Kan" <>,
        David Carrillo Cisneros <>,,,
Subject: Re: [PATCH] fix race caused by hyperthreads when online an offline cpu

On Mon, Jan 16, 2017 at 1:53 AM, zhouchengming
<> wrote:
> On 2017/1/16 17:05, Thomas Gleixner wrote:
>> On Mon, 16 Jan 2017, Zhou Chengming wrote:
>> Can you please stop sending the same patch over and over every other day?
>> Granted, things get forgotten, but sending a polite reminder after a week
>> is definitely enough.
>> Maintainers are not machines responding within a split second on every
>> mail
>> they get. And that patch is not so substantial that it justifies that kind
>> of spam.
> Very sorry for the noise. We are just not sure this is the right fix because
> it's
> hard to reproduce.
I believe this is the right fixed. I tried it and instrumented the
code to verify thread_id
assignment. The problem is easy to reproduce.

$ echo 0 >/sys/devices/system/cpu/cpu2/online
$ echo 1 >/sys/devices/system/cpu/cpu2/online

Normally on Haswell Desktop part,  CPU2 gets thread_id 0 on boot, CPU6
gets thread_id 1.
If you offline CPU2 and bring it back in, it will get thread_id 1 and
thus both sibling will point
to the same exclusive state. The fix is, indeed, to check if the
sibling is not already assigned 1,
and if so to keep 0 for the CPU being online'd.

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