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

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Message-ID: <20110620125715.GK2082@n2100.arm.linux.org.uk>
Date:	Mon, 20 Jun 2011 13:57:15 +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 05:57:01PM +0530, Santosh Shilimkar wrote:
> On 6/20/2011 5:49 PM, Russell King - ARM Linux wrote:
>> On Mon, Jun 20, 2011 at 05:21:48PM +0530, Santosh Shilimkar wrote:
>>> On 6/20/2011 5:10 PM, Russell King - ARM Linux wrote:
>
> [...]
>
>>>
>>> Any pointers on the other question about "why we need to enable
>>> interrupts before the CPU is ready?"
>>
>> To ensure that things like the delay loop calibration and twd calibration
>> can run, though that looks like it'll run happily enough with the boot
>> CPU updating jiffies.
>>
> I guessed it and had same point as above. Calibration will still
> work.
>
>> However, I'm still not taking your patch because I believe its just
>> papering over the real issue, which is not as you describe.
>>
>> You first need to work out why the spinlock lockup detection is firing
>> after just 61us rather than the full 1s and fix that.
>>
> This is possibly because of my script which doesn't wait for 1
> second.

How could a userspace script affect the internal behaviour of
spin_lock() and the spinlock lockup detector?

> Latest mainline kernel with omap2plus_defconfig and below simple script
> to trigger the failure.
>
> -------------
> while true
> do
> echo 0 > /sys/devices/system/cpu/cpu1/online
> echo 1 > /sys/devices/system/cpu/cpu1/online
> done

Thanks, I'll give it a go here and see if I can debug it further.
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