[RFC PATCH] ARM: smp: Fix the CPU hotplug race with scheduler.

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Tue, Jun 21, 2011 at 03:47:04PM +0530, Santosh Shilimkar wrote:
> On 6/21/2011 3:30 PM, Russell King - ARM Linux wrote:
>> On Tue, Jun 21, 2011 at 02:38:34PM +0530, Santosh Shilimkar wrote:
>>> Russell,
>>>
>>> On 6/20/2011 8:24 PM, Santosh Shilimkar wrote:
>>>> On 6/20/2011 7:53 PM, 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?
>>
>> Yes.  I think sending CPUs into infinite loops in the spinlock code is
>> definitely sufficiently serious that it needs to go to Linus ASAP.
>> It'd be nice to have a tested-by line though.
>>
> Sure.
>
>>>> 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 ?
>>
>> I already have that as a separate change.
> Can you point me to both of these commits so that I have
> them in my tree for testing.

I won't be committing the init/calibrate.c change to a git tree - it
isn't ARM stuff so it goes in patch form.