tick: Make oneshot broadcast robust vs. CPU offlining
authorThomas Gleixner <tglx@linutronix.de>
Wed, 26 Jun 2013 10:17:32 +0000 (12:17 +0200)
committerThomas Gleixner <tglx@linutronix.de>
Tue, 2 Jul 2013 12:26:44 +0000 (14:26 +0200)
commitc9b5a266b103af873abb9ac03bc3d067702c8f4b
tree91e5ec118a7e96b998e18a8601d8f14c76d94823
parent47433b8c9d7480a3eebd99df38e857ce85a37cee
tick: Make oneshot broadcast robust vs. CPU offlining

In periodic mode we remove offline cpus from the broadcast propagation
mask. In oneshot mode we fail to do so. This was not a problem so far,
but the recent changes to the broadcast propagation introduced a
constellation which can result in a NULL pointer dereference.

What happens is:

CPU0 CPU1
idle()
  arch_idle()
    tick_broadcast_oneshot_control(OFF);
      set cpu1 in tick_broadcast_force_mask
  if (cpu_offline())
     arch_cpu_dead()

cpu_dead_cleanup(cpu1)
 cpu1 tickdevice pointer = NULL

broadcast interrupt
  dereference cpu1 tickdevice pointer -> OOPS

We dereference the pointer because cpu1 is still set in
tick_broadcast_force_mask and tick_do_broadcast() expects a valid
cpumask and therefor lacks any further checks.

Remove the cpu from the tick_broadcast_force_mask before we set the
tick device pointer to NULL. Also add a sanity check to the oneshot
broadcast function, so we can detect such issues w/o crashing the
machine.

Reported-by: Prarit Bhargava <prarit@redhat.com>
Cc: athorlton@sgi.com
Cc: CAI Qian <caiqian@redhat.com>
Link: http://lkml.kernel.org/r/alpine.DEB.2.02.1306261303260.4013@ionos.tec.linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
kernel/time/tick-broadcast.c