Endless soft-lockups for compiling workload since next-20200519

[Frederic Weisbecker]

On Mon, May 25, 2020 at 03:21:05PM +0200, Peter Zijlstra wrote:
> @@ -2320,7 +2304,7 @@ static void ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
>  
>  	if (llist_add(&p->wake_entry, &rq->wake_list)) {
>  		if (!set_nr_if_polling(rq->idle))
> -			smp_call_function_single_async(cpu, &rq->wake_csd);
> +			smp_call_function_single_async(cpu, &p->wake_csd);
>  		else
>  			trace_sched_wake_idle_without_ipi(cpu);

Ok that's of course very unlikely but could it be possible to have the
following:

CPU 0                         CPU 1                                     CPU 2
-----       

//Wake up A
ttwu_queue(TASK A, CPU 1)     idle_loop {
                                  ttwu_queue_pending {
                                      ....
                                      raw_spin_unlock_irqrestore(rq)
                                      # VMEXIT (with IPI still pending)
                                                                        //task A migrates here
                                                                        wait_event(....)
                                                                        //sleep

//Wake up A
ttwu_queue(TASK A, CPU 2) {
    //IPI on CPU 2 ignored
    // due to csd->flags == CSD_LOCK