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

[Russell King - ARM Linux]

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.

>> 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.