linux-kernel - Re: [PATCH v2] arm: Fix backtrace generation when IPI is masked

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Date:	Tue, 15 Sep 2015 14:15:10 +0100
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Hillf Danton <hillf.zj@...baba-inc.com>
Subject: Re: [PATCH v2] arm: Fix backtrace generation when IPI is masked

On 15/09/15 12:30, Russell King - ARM Linux wrote:
> On Tue, Sep 15, 2015 at 12:05:12PM +0100, Daniel Thompson wrote:
>> Currently on ARM when <SysRq-L> is triggered from an interrupt handler
>> (e.g. a SysRq issued using UART or kbd) the main CPU will wedge for ten
>> seconds with interrupts masked before issuing a backtrace for every CPU
>> except itself.
>>
>> The new backtrace code introduced by commit 96f0e00378d4 ("ARM: add
>> basic support for on-demand backtrace of other CPUs") does not work
>> correctly when run from an interrupt handler because IPI_CPU_BACKTRACE
>> is used to generate the backtrace on all CPUs but cannot preempt the
>> current calling context.
>
> This patch needs a little more work - what happens to the IPI_CPU_BACKTRACE
> we've sent to ourselves?  (It fires after the interrupt handler for the
> UART/kbd has finished.)  It ought to be masked out if we're going to
> handle it a different way.

Actually it already gets masked out. The argument to raise_nmi() points 
to a data structure owned by the backtrace library functions and this 
structure if altered during the execution of nmi_cpu_backtrace() to 
clear the calling CPU.

I had originally planned to use cpumask_test_and_clear_cpu() for the 
conditional branch but that would be broken because nmi_cpu_backtrace() 
would become a nop if we clear anything from the mask before calling it!

I guess I should add a comment about this to save us from broken but 
"obviously correct" cleanups in the future...


Daniel.



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