1 files changed, 54 insertions, 1 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index d8c77bfb6e7e..f3b231cb6b65 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -30,6 +30,7 @@
 #include <linux/relay.h>
 #include <linux/slab.h>
 #include <linux/percpu-rwsem.h>
+#include <linux/cpuset.h>
 
 #include <trace/events/power.h>
 #define CREATE_TRACE_POINTS
@@ -771,7 +772,57 @@ void __init cpuhp_threads_init(void)
 	kthread_unpark(this_cpu_read(cpuhp_state.thread));
 }
 
+/*
+ *
+ * Serialize hotplug trainwrecks outside of the cpu_hotplug_lock
+ * protected region.
+ *
+ * The operation is still serialized against concurrent CPU hotplug via
+ * cpu_add_remove_lock, i.e. CPU map protection.  But it is _not_
+ * serialized against other hotplug related activity like adding or
+ * removing of state callbacks and state instances, which invoke either the
+ * startup or the teardown callback of the affected state.
+ *
+ * This is required for subsystems which are unfixable vs. CPU hotplug and
+ * evade lock inversion problems by scheduling work which has to be
+ * completed _before_ cpu_up()/_cpu_down() returns.
+ *
+ * Don't even think about adding anything to this for any new code or even
+ * drivers. It's only purpose is to keep existing lock order trainwrecks
+ * working.
+ *
+ * For cpu_down() there might be valid reasons to finish cleanups which are
+ * not required to be done under cpu_hotplug_lock, but that's a different
+ * story and would be not invoked via this.
+ */
+static void cpu_up_down_serialize_trainwrecks(bool tasks_frozen)
+{
+	/*
+	 * cpusets delegate hotplug operations to a worker to "solve" the
+	 * lock order problems. Wait for the worker, but only if tasks are
+	 * _not_ frozen (suspend, hibernate) as that would wait forever.
+	 *
+	 * The wait is required because otherwise the hotplug operation
+	 * returns with inconsistent state, which could even be observed in
+	 * user space when a new CPU is brought up. The CPU plug uevent
+	 * would be delivered and user space reacting on it would fail to
+	 * move tasks to the newly plugged CPU up to the point where the
+	 * work has finished because up to that point the newly plugged CPU
+	 * is not assignable in cpusets/cgroups. On unplug that's not
+	 * necessarily a visible issue, but it is still inconsistent state,
+	 * which is the real problem which needs to be "fixed". This can't
+	 * prevent the transient state between scheduling the work and
+	 * returning from waiting for it.
+	 */
+	if (!tasks_frozen)
+		cpuset_wait_for_hotplug();
+}
+
 #ifdef CONFIG_HOTPLUG_CPU
+#ifndef arch_clear_mm_cpumask_cpu
+#define arch_clear_mm_cpumask_cpu(cpu, mm) cpumask_clear_cpu(cpu, mm_cpumask(mm))
+#endif
+
 /**
  * clear_tasks_mm_cpumask - Safely clear tasks' mm_cpumask for a CPU
  * @cpu: a CPU id
@@ -807,7 +858,7 @@ void clear_tasks_mm_cpumask(int cpu)
 		t = find_lock_task_mm(p);
 		if (!t)
 			continue;
-		cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
+		arch_clear_mm_cpumask_cpu(cpu, t->mm);
 		task_unlock(t);
 	}
 	rcu_read_unlock();
@@ -1006,6 +1057,7 @@ out:
 	 */
 	lockup_detector_cleanup();
 	arch_smt_update();
+	cpu_up_down_serialize_trainwrecks(tasks_frozen);
 	return ret;
 }
 
@@ -1141,6 +1193,7 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
 out:
 	cpus_write_unlock();
 	arch_smt_update();
+	cpu_up_down_serialize_trainwrecks(tasks_frozen);
 	return ret;
 }