sched: Make migration_call() safe for stop_machine()-free hotplug

The CPU_DYING branch of migration_call() relies on the fact that
CPU-hotplug offline operations use stop_machine().  This commit therefore
attempts to remedy this situation by acquiring the relevant runqueue
locks.  Note that sched_ttwu_pending() remains outside of the scope of
these new runqueue-lock critical sections because (1) sched_ttwu_pending()
does its own runqueue-lock acquisition and (2) sched_ttwu_pending() handles
pending wakeups, and no further wakeups can select this CPU because it
is already marked as offline.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index eaead2df6aa8f19f70da1b8ae3758bfa15fc2ea2..76d0bf2c1c0d3114a0ddc7f75ee9be76eaa18592 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5175,10 +5175,8 @@ void idle_task_exit(void)
  * their home CPUs. So we just add the counter to another CPU's counter,
  * to keep the global sum constant after CPU-down:
  */
-static void migrate_nr_uninterruptible(struct rq *rq_src)
+static void migrate_nr_uninterruptible(struct rq *rq_src, struct rq *rq_dest)
 {
-       struct rq *rq_dest = cpu_rq(cpumask_any(cpu_active_mask));
-
        rq_dest->nr_uninterruptible += rq_src->nr_uninterruptible;
        rq_src->nr_uninterruptible = 0;
 }
@@ -5200,7 +5198,7 @@ static void calc_global_load_remove(struct rq *rq)
  * there's no concurrency possible, we hold the required locks anyway
  * because of lock validation efforts.
  */
-static void migrate_tasks(unsigned int dead_cpu)
+static void migrate_tasks(unsigned int dead_cpu, struct rq *rq_dest)
 {
        struct rq *rq = cpu_rq(dead_cpu);
        struct task_struct *next, *stop = rq->stop;
@@ -5234,11 +5232,11 @@ static void migrate_tasks(unsigned int dead_cpu)
 
                /* Find suitable destination for @next, with force if needed. */
                dest_cpu = select_fallback_rq(dead_cpu, next);
-               raw_spin_unlock(&rq->lock);
+               double_rq_unlock(rq, rq_dest);
 
                __migrate_task(next, dead_cpu, dest_cpu);
 
-               raw_spin_lock(&rq->lock);
+               double_rq_lock(rq, rq_dest);
        }
 
        rq->stop = stop;
@@ -5452,6 +5450,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
        int cpu = (long)hcpu;
        unsigned long flags;
        struct rq *rq = cpu_rq(cpu);
+       struct rq __maybe_unused *rq_dest = cpu_rq(cpumask_any(cpu_active_mask));
 
        switch (action & ~CPU_TASKS_FROZEN) {
 
@@ -5474,17 +5473,19 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
        case CPU_DYING:
                sched_ttwu_pending();
                /* Update our root-domain */
-               raw_spin_lock_irqsave(&rq->lock, flags);
+               local_irq_save(flags);
+               double_rq_lock(rq, rq_dest);
                if (rq->rd) {
                        BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
                        set_rq_offline(rq);
                }
-               migrate_tasks(cpu);
+               migrate_tasks(cpu, rq_dest);
                BUG_ON(rq->nr_running != 1); /* the migration thread */
-               raw_spin_unlock_irqrestore(&rq->lock, flags);
 
-               migrate_nr_uninterruptible(rq);
+               migrate_nr_uninterruptible(rq, rq_dest);
                calc_global_load_remove(rq);
+               double_rq_unlock(rq, rq_dest);
+               local_irq_restore(flags);
                break;
 #endif
        }