diff --git a/arch/powerpc/platforms/powernv/smp.c b/arch/powerpc/platforms/powernv/smp.c
index 13e25169934665e6655c780def5d3d056775d124..b2ba3e95bda73238b8bcac6de1ea0ba1da3aa9ce 100644
--- a/arch/powerpc/platforms/powernv/smp.c
+++ b/arch/powerpc/platforms/powernv/smp.c
@@ -167,7 +167,6 @@ static void pnv_smp_cpu_kill_self(void)
 	/* Standard hot unplug procedure */
 
 	idle_task_exit();
-	current->active_mm = NULL; /* for sanity */
 	cpu = smp_processor_id();
 	DBG("CPU%d offline\n", cpu);
 	generic_set_cpu_dead(cpu);
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index c49257a3b5108495e7dc36494b3a67e3a2196c65..a132d875d3518ad4fd41a39f2599beece0337222 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -49,6 +49,8 @@ static inline void mmdrop(struct mm_struct *mm)
 		__mmdrop(mm);
 }
 
+void mmdrop(struct mm_struct *mm);
+
 /*
  * This has to be called after a get_task_mm()/mmget_not_zero()
  * followed by taking the mmap_sem for writing before modifying the
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2371292f30b03daa463706a2d9150c9c3bae5861..244d30544377350ae5304a09eafc49ee80bd86db 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3,6 +3,7 @@
  *
  * This code is licenced under the GPL.
  */
+#include <linux/sched/mm.h>
 #include <linux/proc_fs.h>
 #include <linux/smp.h>
 #include <linux/init.h>
@@ -564,6 +565,21 @@ static int bringup_cpu(unsigned int cpu)
 	return bringup_wait_for_ap(cpu);
 }
 
+static int finish_cpu(unsigned int cpu)
+{
+	struct task_struct *idle = idle_thread_get(cpu);
+	struct mm_struct *mm = idle->active_mm;
+
+	/*
+	 * idle_task_exit() will have switched to &init_mm, now
+	 * clean up any remaining active_mm state.
+	 */
+	if (mm != &init_mm)
+		idle->active_mm = &init_mm;
+	mmdrop(mm);
+	return 0;
+}
+
 /*
  * Hotplug state machine related functions
  */
@@ -1549,7 +1565,7 @@ static struct cpuhp_step cpuhp_hp_states[] = {
 	[CPUHP_BRINGUP_CPU] = {
 		.name			= "cpu:bringup",
 		.startup.single		= bringup_cpu,
-		.teardown.single	= NULL,
+		.teardown.single	= finish_cpu,
 		.cant_stop		= true,
 	},
 	/* Final state before CPU kills itself */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2e6ba9e301f031d0941eede35e0a658dc4ccd393..f6ae2629f4e106dfac73b9cd6d63c0279fef6d75 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6197,13 +6197,14 @@ void idle_task_exit(void)
 	struct mm_struct *mm = current->active_mm;
 
 	BUG_ON(cpu_online(smp_processor_id()));
+	BUG_ON(current != this_rq()->idle);
 
 	if (mm != &init_mm) {
 		switch_mm(mm, &init_mm, current);
-		current->active_mm = &init_mm;
 		finish_arch_post_lock_switch();
 	}
-	mmdrop(mm);
+
+	/* finish_cpu(), as ran on the BP, will clean up the active_mm state */
 }
 
 /*