linux-kernel - Re: [PATCH] fix BUG using smp_processor_id() in touch_nmi_watchdog and touch_softlockup

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Message-ID: <AANLkTik-4S_=L5FLhMfommf5Umd0xO+h4QP9uoxwrgFE@mail.gmail.com>
Date:	Tue, 17 Aug 2010 17:05:36 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Don Zickus <dzickus@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix BUG using smp_processor_id() in touch_nmi_watchdog
 and touch_softlockup_watchdog

On Tue, Aug 17, 2010 at 4:39 PM, Sergey Senozhatsky
<sergey.senozhatsky@...il.com> wrote:
>> Why not use __raw_get_cpu_var() instead?
>> You know adding preempt protection in touch_softlockup_watchdog()
>> just suppress the warning. Am I missing something?
>>
>
> Sorry, my low level understanding of the __raw_get_cpu_var isn't very strong.
> I assume it uses current_thread_info()->cpu in some cases (right?) or
> percpu_from_op.

The difference is __raw_get_cpu_var() is using raw_smp_processor_id().

>
>
> Should it be
> acpi_os_stall
>        preepmt_disable
>        touch_nmi_watchdog
>                touch_softlockup_watchdog
>        preempt_enable

Actually I don't think this is helpful for the whole function. Because
if acpi_os_stall()
migrate(I don't know if it could) to another CPU just before
preepmt_disable(), we'll
be on the wrong way. Adding preempt protection is just smoothing the warning.

So I prefer using __raw_get_cpu_var() as what we have been done before.

Thanks,
Yong
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