Re: native_smp_send_reschedule() splat from rt_mutex_lock()?

[Peter Zijlstra]

On Wed, Sep 20, 2017 at 06:24:47PM +0200, Sebastian Andrzej Siewior wrote:
> On 2017-09-18 09:51:10 [-0700], Paul E. McKenney wrote:
> > Hello!
> Hi,
> 
> > [11072.586518] sched: Unexpected reschedule of offline CPU#6!
> > [11072.587578] ------------[ cut here ]------------
> > [11072.588563] WARNING: CPU: 0 PID: 59 at /home/paulmck/public_git/linux-rcu/arch/x86/kernel/smp.c:128 native_smp_send_reschedule+0x37/0x40
> > [11072.591543] Modules linked in:
> > [11072.591543] CPU: 0 PID: 59 Comm: rcub/10 Not tainted 4.14.0-rc1+ #1
> > [11072.610596] Call Trace:
> > [11072.611531]  resched_curr+0x61/0xd0
> > [11072.611531]  switched_to_rt+0x8f/0xa0
> > [11072.612647]  rt_mutex_setprio+0x25c/0x410
> > [11072.613591]  task_blocks_on_rt_mutex+0x1b3/0x1f0
> > [11072.614601]  rt_mutex_slowlock+0xa9/0x1e0
> > [11072.615567]  rt_mutex_lock+0x29/0x30
> > [11072.615567]  rcu_boost_kthread+0x127/0x3c0
> 
> > In theory, I could work around this by excluding CPU-hotplug operations
> > while doing RCU priority boosting, but in practice I am very much hoping
> > that there is a more reasonable solution out there...
> 
> so in CPUHP_TEARDOWN_CPU / take_cpu_down() / __cpu_disable() the CPU is
> marked as offline and interrupt handling is disabled. Later in
> CPUHP_AP_SCHED_STARTING / sched_cpu_dying() all tasks are migrated away.
> 
> Did this hit a random task during a CPU-hotplug operation which was not
> yet migrated away from the dying CPU? In theory a futex_unlock() of a RT
> task could also produce such a backtrace.

So this is an interrupt that got received while we were going down, and
processed after we've migrated the tasks away, right?

Should we not clear the IRQ pending masks somewhere along the line?

Other than that, there's nothing we can do to avoid this race.