workqueues: add comments to __create_workqueue_key()

Dmitry Adamushko pointed out that the error handling in
__create_workqueue_key() is not clear, add the comment.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index ec7e4f62aaff..4a26a1382df0 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -830,10 +830,21 @@ struct workqueue_struct *__create_workqueue_key(const char *name,
                 start_workqueue_thread(cwq, -1);
         } else {
                 cpu_maps_update_begin();
+                /*
+                 * We must place this wq on list even if the code below fails.
+                 * cpu_down(cpu) can remove cpu from cpu_populated_map before
+                 * destroy_workqueue() takes the lock, in that case we leak
+                 * cwq[cpu]->thread.
+                 */
                 spin_lock(&workqueue_lock);
                 list_add(&wq->list, &workqueues);
                 spin_unlock(&workqueue_lock);
-
+                /*
+                 * We must initialize cwqs for each possible cpu even if we
+                 * are going to call destroy_workqueue() finally. Otherwise
+                 * cpu_up() can hit the uninitialized cwq once we drop the
+                 * lock.
+                 */
                 for_each_possible_cpu(cpu) {
                         cwq = init_cpu_workqueue(wq, cpu);
                         if (err || !cpu_online(cpu))