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Subject[tip:timers/core] tick: Make oneshot broadcast robust vs. CPU offlining
Commit-ID:  c9b5a266b103af873abb9ac03bc3d067702c8f4b
Gitweb: http://git.kernel.org/tip/c9b5a266b103af873abb9ac03bc3d067702c8f4b
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Wed, 26 Jun 2013 12:17:32 +0200
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 2 Jul 2013 14:26:44 +0200

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 | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index d067c01..4790037 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -533,6 +533,13 @@ again:
cpumask_clear(tick_broadcast_force_mask);

/*
+ * Sanity check. Catch the case where we try to broadcast to
+ * offline cpus.
+ */
+ if (WARN_ON_ONCE(!cpumask_subset(tmpmask, cpu_online_mask)))
+ cpumask_and(tmpmask, tmpmask, cpu_online_mask);
+
+ /*
* Wakeup the cpus which have an expired event.
*/
tick_do_broadcast(tmpmask);
@@ -773,10 +780,12 @@ void tick_shutdown_broadcast_oneshot(unsigned int *cpup)
raw_spin_lock_irqsave(&tick_broadcast_lock, flags);

/*
- * Clear the broadcast mask flag for the dead cpu, but do not
- * stop the broadcast device!
+ * Clear the broadcast masks for the dead cpu, but do not stop
+ * the broadcast device!
*/
cpumask_clear_cpu(cpu, tick_broadcast_oneshot_mask);
+ cpumask_clear_cpu(cpu, tick_broadcast_pending_mask);
+ cpumask_clear_cpu(cpu, tick_broadcast_force_mask);

raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
}

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