linux-kernel - Re: Re: [patch] ARM: smpboot: Enable interrupts after marking CPU online/active

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Message-ID: <CAPz4a6BwzAmE=08pCfhK5of0vEV+uPaf8D-9U_MnW5_vAe6NFw@mail.gmail.com>
Date:	Tue, 13 Dec 2011 16:13:30 -0800
From:	Dima Zavin <dima@...roid.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Stepan Moskovchenko <stepanm@...eaurora.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Frank Rowand <frank.rowand@...sony.com>,
	amit kachhap <amit.kachhap@...aro.org>,
	Colin Cross <ccross@...roid.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	chaos.youn@...sung.com, LAK <linux-arm-kernel@...ts.infradead.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Re: [patch] ARM: smpboot: Enable interrupts after marking CPU online/active

Thomas,

Did you ever get a chance to look into this further?

Thanks in advance.

--Dima

On Tue, Nov 15, 2011 at 2:00 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 15 Nov 2011, Stepan Moskovchenko wrote:
>> I am seeing a deadlock when executing hotplug operations with this patch
>> applied. When the secondary CPU gets brought up in _cpu_up, the cpu is turned
>> on
>> and then the online notifier gets called, which is what marks the secondary
>> CPU
>> as active. If _cpu_up on the primary CPU is preempted before the secondary CPU
>> is marked active, it is possible that the primary CPU will want to call
>> smp_call_function (or send an IPI) to the secondary CPU because it is marked
>> online. However, with this patch, the secondary CPU is still spinning on
>> !cpu_active(cpu)
>> with interrupts disabled. So, the primary CPU is now stuck in csd_lock_wait(),
>> waiting for the secondary CPU to respond, while the secondary CPU spins with
>> interrupts disabled, waiting for the primary CPU to mark it as active. So,
>> while
>> your approach to not call smp_function_single may work for you in your
>> specific
>> case, I believe there is still a problem in the general case.
>>
>> One suggestion for resolving this might be making smp_call_function look at
>> the
>> active CPUs rather than online CPUs, or to just let the secondary CPU mark
>> itself as active rather than having the primary CPU do this, though this might
>> defeat the original intended purpose of the active mask.
>
> What a mess. I'll have a look tomorrow.
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