linux-kernel - Re: [RFC PATCH] ARM: smp: Fix the CPU hotplug race with scheduler.

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Message-ID: <20110620103521.GE2082@n2100.arm.linux.org.uk>
Date:	Mon, 20 Jun 2011 11:35:21 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Santosh Shilimkar <santosh.shilimkar@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH] ARM: smp: Fix the CPU hotplug race with scheduler.

On Mon, Jun 20, 2011 at 03:58:03PM +0530, Santosh Shilimkar wrote:
> On 6/20/2011 3:44 PM, Russell King - ARM Linux wrote:
>> On Mon, Jun 20, 2011 at 10:50:53AM +0100, Russell King - ARM Linux wrote:
>>> On Mon, Jun 20, 2011 at 02:53:59PM +0530, Santosh Shilimkar wrote:
>>>> The current ARM CPU hotplug code suffers from couple of race conditions
>>>> in CPU online path with scheduler.
>>>> The ARM CPU hotplug code doesn't wait for hot-plugged CPU to be marked
>>>> active as part of cpu_notify() by the CPU which brought it up before
>>>> enabling interrupts.
>>>
>>> Hmm, why not just move the set_cpu_online() call before notify_cpu_starting()
>>> and add the wait after the set_cpu_online() ?
>>
>> Actually, the race is caused by the CPU being marked online (and therefore
>> available for the scheduler) but not yet active (the CPU asking this one
>> to boot hasn't run the online notifiers yet.)
>>
> Scheduler uses the active mask and not online mask. For schedules CPU
> is ready for migration as soon as it is marked as active and that's
> the reason, interrupts should never be enabled before CPU is marked
> as active in online path.
>
>> This, I feel, is a fault of generic code.  If the CPU is not ready to have
>> processes scheduled on it (because migration is not initialized) then we
>> shouldn't be scheduling processes on the new CPU yet.
>>
>> In any case, this should close the window by ensuring that we don't receive
>> an interrupt in the online-but-not-active case.  Can you please test?
>>
> No it doesn't work. I still get the crash. The important point
> here is not to enable interrupts before CPU is marked
> as online and active.

But we can't do that.
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