io-wq: check for wq exit after adding new worker task_work

We check IO_WQ_BIT_EXIT before attempting to create a new worker, and
wq exit cancels pending work if we have any. But it's possible to have
a race between the two, where creation checks exit finding it not set,
but we're in the process of exiting. The exit side will cancel pending
creation task_work, but there's a gap where we add task_work after we've
canceled existing creations at exit time.

Fix this by checking the EXIT bit post adding the creation task_work.
If it's set, run the same cancelation that exit does.

Reported-and-tested-by: syzbot+b60c982cb0efc5e05a47@syzkaller.appspotmail.com
Reviewed-by: Hao Xu <haoxu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

fs/io-wq.c

@@ -142,6 +142,7 @@ static bool io_acct_cancel_pending_work(struct io_wqe *wqe,
 					struct io_wqe_acct *acct,
 					struct io_cb_cancel_data *match);
 static void create_worker_cb(struct callback_head *cb);
+static void io_wq_cancel_tw_create(struct io_wq *wq);
 
 static bool io_worker_get(struct io_worker *worker)
 {
@@ -357,10 +358,22 @@ static bool io_queue_worker_create(struct io_worker *worker,
 	    test_and_set_bit_lock(0, &worker->create_state))
 		goto fail_release;
 
+	atomic_inc(&wq->worker_refs);
 	init_task_work(&worker->create_work, func);
 	worker->create_index = acct->index;
-	if (!task_work_add(wq->task, &worker->create_work, TWA_SIGNAL))
+	if (!task_work_add(wq->task, &worker->create_work, TWA_SIGNAL)) {
+		/*
+		 * EXIT may have been set after checking it above, check after
+		 * adding the task_work and remove any creation item if it is
+		 * now set. wq exit does that too, but we can have added this
+		 * work item after we canceled in io_wq_exit_workers().
+		 */
+		if (test_bit(IO_WQ_BIT_EXIT, &wq->state))
+			io_wq_cancel_tw_create(wq);
+		io_worker_ref_put(wq);
 		return true;
+	}
+	io_worker_ref_put(wq);
 	clear_bit_unlock(0, &worker->create_state);
 fail_release:
 	io_worker_release(worker);
@@ -1196,20 +1209,26 @@ void io_wq_exit_start(struct io_wq *wq)
 	set_bit(IO_WQ_BIT_EXIT, &wq->state);
 }
 
-static void io_wq_exit_workers(struct io_wq *wq)
+static void io_wq_cancel_tw_create(struct io_wq *wq)
 {
 	struct callback_head *cb;
-	int node;
-
-	if (!wq->task)
-		return;
 
 	while ((cb = task_work_cancel_match(wq->task, io_task_work_match, wq)) != NULL) {
 		struct io_worker *worker;
 
 		worker = container_of(cb, struct io_worker, create_work);
 		io_worker_cancel_cb(worker);
 	}
+}
+
+static void io_wq_exit_workers(struct io_wq *wq)
+{
+	int node;
+
+	if (!wq->task)
+		return;
+
+	io_wq_cancel_tw_create(wq);
 
 	rcu_read_lock();
 	for_each_node(node) {