On 6/20/2011 4:15 PM, Santosh Shilimkar wrote:
On 6/20/2011 4:05 PM, Russell King - ARM Linux wrote:
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.
Why is that ?
Is it because of calibration or the hotplug start notifies needs to
be called with interrupts enabled ?
BTW, how is ARM different from X86 here. I mean the X86 code seems to
do similar what my patch is trying to fix for ARM. Some pointers
would help me to understand why can't we delay the interrupt enable
part on ARM hotplug code.
Regards
Santosh
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