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

[Jonas Jelonek <jelonek.jonas@gmail.com>]

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
91840be8f710 ("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 popped up during kernel bump downstream in OpenWrt from
6.18.33 to 6.18.34, since the suspected change has been backported to
6.18 stable branch [1].

Call rcutree_report_cpu_dead() once interrupts are disabled, mirroring the
generic CPU-hotplug offline path (and arm64's stop handling), so RCU stops
waiting on the parked CPUs and grace periods can still complete.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-6.18.y&id=18c0456ea2615b1a743a6db739c74411c3b42bc6

Fixes: 91840be8f710 ("irq_work: Fix use-after-free in irq_work_single() on PREEMPT_RT")
CC: stable@vger.kernel.org
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>

diff --git a/arch/mips/kernel/smp.c b/arch/mips/kernel/smp.c
index 4868e79f3b30..0f28b4a62e72 100644
--- a/arch/mips/kernel/smp.c
+++ b/arch/mips/kernel/smp.c
@@ -20,6 +20,7 @@
 #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>
@@ -422,6 +423,7 @@ static void stop_this_cpu(void *dummy)
 	set_cpu_online(smp_processor_id(), false);
 	calculate_cpu_foreign_map();
 	local_irq_disable();
+	rcutree_report_cpu_dead();
 	while (1);
 }
 
-- 
2.51.0