sched/rt: Skip currently executing CPU in rto_next_cpu()

In the Linux kernel, the following vulnerability has been resolved:

sched/rt: Skip currently executing CPU in rto_next_cpu()

CPU0 becomes overloaded when hosting a CPU-bound RT task, a non-CPU-bound

RT task, and a CFS task stuck in kernel space. When other CPUs switch from

RT to non-RT tasks, RT load balancing (LB) is triggered; with

HAVE_RT_PUSH_IPI enabled, they send IPIs to CPU0 to drive the execution

of rto_push_irq_work_func. During push_rt_task on CPU0,

if next_task->prio < rq->donor->prio, resched_curr() sets NEED_RESCHED

and after the push operation completes, CPU0 calls rto_next_cpu().

Since only CPU0 is overloaded in this scenario, rto_next_cpu() should

ideally return -1 (no further IPI needed).

However, multiple CPUs invoking tell_cpu_to_push() during LB increments

rd->rto_loop_next. Even when rd->rto_cpu is set to -1, the mismatch between

rd->rto_loop and rd->rto_loop_next forces rto_next_cpu() to restart its

search from -1. With CPU0 remaining overloaded (satisfying rt_nr_migratory

&& rt_nr_total > 1), it gets reselected, causing CPU0 to queue irq_work to

itself and send self-IPIs repeatedly. As long as CPU0 stays overloaded and

other CPUs run pull_rt_tasks(), it falls into an infinite self-IPI loop,

which triggers a CPU hardlockup due to continuous self-interrupts.

The trigging scenario is as follows:

cpu0 cpu1 cpu2

pull_rt_task

tell_cpu_to_push

<------------irq_work_queue_on

rto_push_irq_work_func

push_rt_task

resched_curr(rq) pull_rt_task

rto_next_cpu tell_cpu_to_push

<-------------------------- atomic_inc(rto_loop_next)

rd->rto_loop != next

rto_next_cpu

irq_work_queue_on

rto_push_irq_work_func

Fix redundant self-IPI by filtering the initiating CPU in rto_next_cpu().

This solution has been verified to effectively eliminate spurious self-IPIs

and prevent CPU hardlockup scenarios.