[PATCH 2/2] hrtimer: Prevent enqueue of hrtimer on dead CPU

[Michael Bohan]

When switching the hrtimer cpu_base, we briefly allow for
preemption to become enabled by unlocking the cpu_base lock.
During this time, the CPU corresponding to the new cpu_base
that was selected may in fact go offline. In this scenario, the
hrtimer is enqueued to a CPU that's not online, and therefore
it never fires.

As an example, consider this example:

CPU #0                          CPU #1
----                            ----
...                             hrtimer_start()
                                 lock_hrtimer_base()
                                 switch_hrtimer_base()
                                  cpu = hrtimer_get_target() -> 1
                                  spin_unlock(&cpu_base->lock)
                                <migrate thread to CPU #0>
                                <offline>
spin_lock(&new_base->lock)
this_cpu = 0
cpu != this_cpu
enqueue_hrtimer(cpu_base #1)

To prevent this scenario, verify that the CPU corresponding to
the new cpu_base is indeed online before selecting it in
hrtimer_switch_base(). If it's not online, fallback to using the
base of the current CPU.

Signed-off-by: Michael Bohan <mbohan@xxxxxxxxxxxxxx>
---
 kernel/hrtimer.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 3f0bce9..54c5d98 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -227,7 +227,8 @@ again:
 
 		this_cpu = smp_processor_id();
 
-		if (cpu != this_cpu && hrtimer_check_target(timer, new_base)) {
+		if (cpu != this_cpu && (hrtimer_check_target(timer, new_base)
+			|| !cpu_online(cpu))) {
 			raw_spin_unlock(&new_base->cpu_base->lock);
 			raw_spin_lock(&base->cpu_base->lock);
 			cpu = smp_processor_id();
-- 
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