MIPS: smp: report dying CPU to RCU in stop_this_cpu()

commit 9f3f3bd upstream.

smp_send_stop() parks all secondary CPUs in stop_this_cpu(). The function
marks the CPU offline for the scheduler via set_cpu_online(false) but
never informs RCU, so RCU keeps expecting a quiescent state from CPUs
that are now spinning forever with interrupts disabled.

As long as nothing waits for an RCU grace period after smp_send_stop()
this is harmless, which is why it went unnoticed. Since commit
91840be ("irq_work: Fix use-after-free in irq_work_single() on PREEMPT_RT")
however, irq_work_sync() calls synchronize_rcu() on architectures without
an irq_work self-IPI, i.e. where arch_irq_work_has_interrupt() returns
false. That is the asm-generic default used by MIPS. Any irq_work_sync()
issued in the reboot/shutdown path after smp_send_stop() then blocks on
a grace period that can never complete, hanging the reboot:

  WARNING: CPU: 0 PID: 15 at kernel/irq_work.c:144 irq_work_queue_on
  ...
  rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
  rcu: Offline CPU 1 blocking current GP.
  rcu: Offline CPU 2 blocking current GP.
  rcu: Offline CPU 3 blocking current GP.

This issue was noticed on several Realtek MIPS switch SoCs (MIPS
interAptiv) and came up during kernel bump downstream in OpenWrt from
6.18.33 to 6.18.34, after the backport of the patch to the 6.18 stable
branch. The patch also has been backported all the way back to 6.1.

Call rcu_report_dead() once interrupts are disabled, mirroring the
generic CPU-hotplug offline path, so RCU stops waiting on the parked CPUs
and grace periods can still complete. MIPS shuts down all CPUs here
without going through the CPU-hotplug mechanism, so this report is not
otherwise issued. Reporting a dying CPU to RCU outside the regular hotplug
offline path is not unprecedented: arm64 does the same in cpu_die_early().
There it is an exception for a CPU that was coming online and is aborting
bringup, rather than the default shutdown action as on MIPS.

Note: this differs from the upstream commit in that it calls
rcu_report_dead(smp_processor_id()) instead of rcutree_report_cpu_dead().
The latter, along with the rcutree_report_cpu_dead() name, was introduced
by commit 448e9f3 ("rcu: Standardize explicit CPU-hotplug calls")
in v6.7; on this kernel the equivalent function is still named
rcu_report_dead() and takes an explicit CPU argument.

Fixes: 91840be ("irq_work: Fix use-after-free in irq_work_single() on PREEMPT_RT")
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: jonasjelonek

arch/mips/kernel/smp.c

@@ -19,6 +19,7 @@
 #include <linux/sched/mm.h>
 #include <linux/cpumask.h>
 #include <linux/cpu.h>
+#include <linux/rcupdate.h>
 #include <linux/err.h>
 #include <linux/ftrace.h>
 #include <linux/irqdomain.h>
@@ -410,6 +411,7 @@ static void stop_this_cpu(void *dummy)
 	set_cpu_online(smp_processor_id(), false);
 	calculate_cpu_foreign_map();
 	local_irq_disable();
+	rcu_report_dead(smp_processor_id());
 	while (1);
 }