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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