[PATCH RT 5/9] kernel/hrtimer/hotplug: dont wake ktimersoftd while holding the hrtimer base lock

[Steven Rostedt]

4.4.102-rt117-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Mike Galbraith <efa...@gmx.de>

kernel/hrtimer: don't wakeup a process while holding the hrtimer base lock
missed a path, namely hrtimers_dead_cpu() -> migrate_hrtimer_list().  Defer
raising softirq until after base lock has been released there as well.

Signed-off-by: Mike Galbraith <efa...@gmx.de>
Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
---
 kernel/time/hrtimer.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 78066f7ea2a2..120fc8932165 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1857,7 +1857,7 @@ static void init_hrtimers_cpu(int cpu)
 
 #ifdef CONFIG_HOTPLUG_CPU
 
-static void migrate_hrtimer_list(struct hrtimer_clock_base *old_base,
+static int migrate_hrtimer_list(struct hrtimer_clock_base *old_base,
                                struct hrtimer_clock_base *new_base)
 {
        struct hrtimer *timer;
@@ -1887,15 +1887,19 @@ static void migrate_hrtimer_list(struct 
hrtimer_clock_base *old_base,
        }
 #ifdef CONFIG_PREEMPT_RT_BASE
        list_splice_tail(&old_base->expired, &new_base->expired);
-       if (!list_empty(&new_base->expired))
-               raise_softirq_irqoff(HRTIMER_SOFTIRQ);
+       /*
+        * Tell the caller to raise HRTIMER_SOFTIRQ.  We can't safely
+        * acquire ktimersoftd->pi_lock while the base lock is held.
+        */
+       return !list_empty(&new_base->expired);
 #endif
+       return 0;
 }
 
 static void migrate_hrtimers(int scpu)
 {
        struct hrtimer_cpu_base *old_base, *new_base;
-       int i;
+       int i, raise = 0;
 
        BUG_ON(cpu_online(scpu));
        tick_cancel_sched_timer(scpu);
@@ -1911,13 +1915,16 @@ static void migrate_hrtimers(int scpu)
        raw_spin_lock_nested(&old_base->lock, SINGLE_DEPTH_NESTING);
 
        for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
-               migrate_hrtimer_list(&old_base->clock_base[i],
-                                    &new_base->clock_base[i]);
+               raise |= migrate_hrtimer_list(&old_base->clock_base[i],
+                                             &new_base->clock_base[i]);
        }
 
        raw_spin_unlock(&old_base->lock);
        raw_spin_unlock(&new_base->lock);
 
+       if (raise)
+               raise_softirq_irqoff(HRTIMER_SOFTIRQ);
+
        /* Check, if we got expired work to do */
        __hrtimer_peek_ahead_timers();
        local_irq_enable();
-- 
2.13.2