workqueue: Fix pwq ref leak in rescuer_thread()

BugLink: https://bugs.launchpad.net/bugs/1858424
commit e66b39af00f426b3356b96433d620cb3367ba1ff upstream.

008847f66c3 ("workqueue: allow rescuer thread to do more work.") made
the rescuer worker requeue the pwq immediately if there may be more
work items which need rescuing instead of waiting for the next mayday
timer expiration.  Unfortunately, it doesn't check whether the pwq is
already on the mayday list and unconditionally gets the ref and moves
it onto the list.  This doesn't corrupt the list but creates an
additional reference to the pwq.  It got queued twice but will only be
removed once.

This leak later can trigger pwq refcnt warning on workqueue
destruction and prevent freeing of the workqueue.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "Williams, Gerald S" <gerald.s.williams@intel.com>
Cc: NeilBrown <neilb@suse.de>
Cc: stable@vger.kernel.org # v3.19+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 93e20f5330fcbbccf70821d496377fd7b79f9886..02319b84ef681836f0275113a05818699902988c 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2532,8 +2532,14 @@ repeat:
                         */
                        if (need_to_create_worker(pool)) {
                                spin_lock(&wq_mayday_lock);
-                               get_pwq(pwq);
-                               list_move_tail(&pwq->mayday_node, &wq->maydays);
+                               /*
+                                * Queue iff we aren't racing destruction
+                                * and somebody else hasn't queued it already.
+                                */
+                               if (wq->rescuer && list_empty(&pwq->mayday_node)) {
+                                       get_pwq(pwq);
+                                       list_add_tail(&pwq->mayday_node, &wq->maydays);
+                               }
                                spin_unlock(&wq_mayday_lock);
                        }
                }
@@ -4652,7 +4658,8 @@ static void show_pwq(struct pool_workqueue *pwq)
        pr_info("  pwq %d:", pool->id);
        pr_cont_pool_info(pool);
 
-       pr_cont(" active=%d/%d%s\n", pwq->nr_active, pwq->max_active,
+       pr_cont(" active=%d/%d refcnt=%d%s\n",
+               pwq->nr_active, pwq->max_active, pwq->refcnt,
                !list_empty(&pwq->mayday_node) ? " MAYDAY" : "");
 
        hash_for_each(pool->busy_hash, bkt, worker, hentry) {