Re: CFS scheduler: spin_lock usage causes dead lock when smp_apic_timer_interrupt occurs

[Mike Galbraith]

On Mon, 2019-01-07 at 11:26 +0100, Peter Zijlstra wrote:
> 
> I would expect lockdep you also complain about this...

And grumble it did.

commit df7e8acc0c9a84979a448d215b8ef889efe4ac5a
Author: Mike Galbraith <efault@xxxxxx>
Date:   Fri May 4 08:14:38 2018 +0200

    sched/fair: Fix CFS bandwidth control lockdep DEADLOCK report
    
    CFS bandwidth control yields the inversion gripe below, moving
    handling quells it.
    
    |========================================================
    |WARNING: possible irq lock inversion dependency detected
    |4.16.7-rt1-rt #2 Tainted: G            E
    |--------------------------------------------------------
    |sirq-hrtimer/0/15 just changed the state of lock:
    | (&cfs_b->lock){+...}, at: [<000000009adb5cf7>] sched_cfs_period_timer+0x28/0x140
    |but this lock was taken by another, HARDIRQ-safe lock in the past: (&rq->lock){-...}
    |and interrupts could create inverse lock ordering between them.
    |other info that might help us debug this:
    | Possible interrupt unsafe locking scenario:
    |       CPU0                    CPU1
    |       ----                    ----
    |  lock(&cfs_b->lock);
    |                               local_irq_disable();
    |                               lock(&rq->lock);
    |                               lock(&cfs_b->lock);
    |  <Interrupt>
    |    lock(&rq->lock);
    
    Cc: stable-rt@xxxxxxxxxxxxxxx
    Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
    Signed-off-by: Mike Galbraith <efault@xxxxxx>
    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 960ad0ce77d7..420624c49f38 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5007,9 +5007,9 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
 	cfs_b->period = ns_to_ktime(default_cfs_period());
 
 	INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq);
-	hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);
+	hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED_HARD);
 	cfs_b->period_timer.function = sched_cfs_period_timer;
-	hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	cfs_b->slack_timer.function = sched_cfs_slack_timer;
 }