kthreads: simplify migration_thread() exit path

Now that kthread_stop() can be used even if the task has already exited,
we can kill the "wait_to_die:" loop in migration_thread().  But we must
pin rq->migration_thread after creation.

Actually, I don't think CPU_UP_CANCELED or CPU_DEAD should wait for
->migration_thread exit.  Perhaps we can simplify this code a bit more.
migration_call() can set ->should_stop and forget about this thread.  But
we need a new helper in kthred.c for that.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Vitaliy Gusev <vgusev@openvz.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 10 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 8fb88a906aaa..247fd0fedd0b 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -7045,7 +7045,7 @@ static int migration_thread(void *data)
 
                 if (cpu_is_offline(cpu)) {
                         spin_unlock_irq(&rq->lock);
-                        goto wait_to_die;
+                        break;
                 }
 
                 if (rq->active_balance) {
@@ -7071,16 +7071,7 @@ static int migration_thread(void *data)
                 complete(&req->done);
         }
         __set_current_state(TASK_RUNNING);
-        return 0;
 
-wait_to_die:
-        /* Wait for kthread_stop */
-        set_current_state(TASK_INTERRUPTIBLE);
-        while (!kthread_should_stop()) {
-                schedule();
-                set_current_state(TASK_INTERRUPTIBLE);
-        }
-        __set_current_state(TASK_RUNNING);
         return 0;
 }
 
@@ -7494,6 +7485,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
                 rq = task_rq_lock(p, &flags);
                 __setscheduler(rq, p, SCHED_FIFO, MAX_RT_PRIO-1);
                 task_rq_unlock(rq, &flags);
+                get_task_struct(p);
                 cpu_rq(cpu)->migration_thread = p;
                 break;
 
@@ -7524,6 +7516,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
                 kthread_bind(cpu_rq(cpu)->migration_thread,
                              cpumask_any(cpu_online_mask));
                 kthread_stop(cpu_rq(cpu)->migration_thread);
+                put_task_struct(cpu_rq(cpu)->migration_thread);
                 cpu_rq(cpu)->migration_thread = NULL;
                 break;
 
@@ -7533,6 +7526,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
                 migrate_live_tasks(cpu);
                 rq = cpu_rq(cpu);
                 kthread_stop(rq->migration_thread);
+                put_task_struct(rq->migration_thread);
                 rq->migration_thread = NULL;
                 /* Idle task back to normal (off runqueue, low prio) */
                 spin_lock_irq(&rq->lock);