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

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Date:	Tue, 21 Jun 2011 14:38:34 +0530
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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.

Russell,

On 6/20/2011 8:24 PM, Santosh Shilimkar wrote:
> On 6/20/2011 7:53 PM, Russell King - ARM Linux wrote:
>> On Mon, Jun 20, 2011 at 12:40:19PM +0100, Russell King - ARM Linux wrote:
[....]

>> So, as loops_per_jiffy is not local to this function, the compiler has
>> to write out that zero value, before calling calibrate_delay_converge(),
>> and loops_per_jiffy only becomes non-zero _after_
>> calibrate_delay_converge()
>> has returned. This opens the window and allows the spinlock debugging
>> code to explode.
>>
>> This patch closes the window completely, by only writing to
>> loops_per_jiffy
>> only when we have a real value for it.
>>
>> This allows me to boot 3.0.0-rc3 on Versatile Express (4 CPU) whereas
>> without this it fails with spinlock lockup and rcu problems.
>>
>> init/calibrate.c | 14 ++++++++------
>> 1 files changed, 8 insertions(+), 6 deletions(-)
>>
> I am away from my board now. Will test this change.
Have tested your change and it seems to fix the crash I
was observing. Are you planning to send this fix for rc5?

> btw, the online-active race is still open even with this patch close
> and should be fixed.
>
The only problem remains is waiting for active mask before
marking CPU online. Shall I refresh my patch with only
this change then ?

Regards
Santosh
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