io-wq: fix no lock protection of acct->nr_worker

There is an acct->nr_worker visit without lock protection. Think about
the case: two callers call io_wqe_wake_worker(), one is the original
context and the other one is an io-worker(by calling
io_wqe_enqueue(wqe, linked)), on two cpus paralelly, this may cause
nr_worker to be larger than max_worker.
Let's fix it by adding lock for it, and let's do nr_workers++ before
create_io_worker. There may be a edge cause that the first caller fails
to create an io-worker, but the second caller doesn't know it and then
quit creating io-worker as well:

say nr_worker = max_worker - 1
        cpu 0                        cpu 1
   io_wqe_wake_worker()          io_wqe_wake_worker()
      nr_worker < max_worker
      nr_worker++
      create_io_worker()         nr_worker == max_worker
         failed                  return
      return

But the chance of this case is very slim.

Fixes: 685fe7fe ("io-wq: eliminate the need for a manager thread")
Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
[axboe: fix unconditional create_io_worker() call]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -247,9 +247,18 @@ static void io_wqe_wake_worker(struct io_wqe *wqe, struct io_wqe_acct *acct)
 	ret = io_wqe_activate_free_worker(wqe);
 	rcu_read_unlock();
 
-	if (!ret && acct->nr_workers < acct->max_workers) {
+	if (!ret) {
+		bool do_create = false;
+
+		raw_spin_lock_irq(&wqe->lock);
+		if (acct->nr_workers < acct->max_workers) {
 			atomic_inc(&acct->nr_running);
 			atomic_inc(&wqe->wq->worker_refs);
+			acct->nr_workers++;
+			do_create = true;
+		}
+		raw_spin_unlock_irq(&wqe->lock);
+		if (do_create)
 			create_io_worker(wqe->wq, wqe, acct->index);
 	}
 }
@@ -635,6 +644,9 @@ static void create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index)
 		kfree(worker);
 fail:
 		atomic_dec(&acct->nr_running);
+		raw_spin_lock_irq(&wqe->lock);
+		acct->nr_workers--;
+		raw_spin_unlock_irq(&wqe->lock);
 		io_worker_ref_put(wq);
 		return;
 	}
@@ -650,9 +662,8 @@ static void create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index)
 	worker->flags |= IO_WORKER_F_FREE;
 	if (index == IO_WQ_ACCT_BOUND)
 		worker->flags |= IO_WORKER_F_BOUND;
-	if (!acct->nr_workers && (worker->flags & IO_WORKER_F_BOUND))
+	if ((acct->nr_workers == 1) && (worker->flags & IO_WORKER_F_BOUND))
 		worker->flags |= IO_WORKER_F_FIXED;
-	acct->nr_workers++;
 	raw_spin_unlock_irq(&wqe->lock);
 	wake_up_new_task(tsk);
 }