From mboxrd@z Thu Jan  1 00:00:00 1970
From: daniel.thompson@linaro.org (Daniel Thompson)
Date: Tue, 15 Sep 2015 14:15:10 +0100
Subject: [PATCH v2] arm: Fix backtrace generation when IPI is masked
In-Reply-To: <20150915113003.GS21084@n2100.arm.linux.org.uk>
References: <1442222294-2756-1-git-send-email-daniel.thompson@linaro.org>
 <1442315112-14039-1-git-send-email-daniel.thompson@linaro.org>
 <20150915113003.GS21084@n2100.arm.linux.org.uk>
Message-ID: <55F819DE.9040704@linaro.org>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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.