Just got my 6 way machine to a state where cpu 0 is in an
endless loop within __smp_call_function_single.
All other cpus are idle.

The call trace on cpu 0 looks like this:

 __smp_call_function_single
 scheduler_tick
 update_process_times
 tick_sched_timer
 __run_hrtimer
 hrtimer_interrupt
 clock_comparator_work
 do_extint
 ext_int_handler
 ----> timer irq
 cpu_idle

__smp_call_function_single() got called from nohz_balancer_kick()
(inlined) with the remote cpu being 1, wait being 0 and the per
cpu variable remote_sched_softirq_cb (call_single_data) of the
current cpu (0).

Then it loops forever when it tries to grab the lock of the
call_single_data, since it is already locked and enqueued on cpu 0.

My theory how this could have happened: for some reason the
scheduler decided to call __smp_call_function_single() on it's own
cpu, and sends an IPI to itself. The interrupt stays pending
since IRQs are disabled. If then the hypervisor schedules the
cpu away it might happen that upon rescheduling both the IPI and
the timer IRQ are pending. If then interrupts are enabled again
it depends which one gets scheduled first.
If the timer interrupt gets delivered first we end up with the
local deadlock as seen in the calltrace above.

Let's make __smp_call_function_single() check if the target cpu is
the current cpu and execute the function immediately just like
smp_call_function_single does. That should prevent at least the
scenario described here.

It might also be that the scheduler is not supposed to call
__smp_call_function_single with the remote cpu being the current
cpu, but that is a different issue.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Jens Axboe <jaxboe@fusionio.com>
Cc: Venkatesh Pallipadi <venki@google.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <20100910114729.GB2827@osiris.boeblingen.de.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>