io-wq: fix IO_WORKER_F_FIXED issue in create_io_worker()

There may be cases like:
        A                                 B
spin_lock(wqe->lock)
nr_workers is 0
nr_workers++
spin_unlock(wqe->lock)
                                     spin_lock(wqe->lock)
                                     nr_wokers is 1
                                     nr_workers++
                                     spin_unlock(wqe->lock)
create_io_worker()
  acct->worker is 1
                                     create_io_worker()
                                       acct->worker is 1

There should be one worker marked IO_WORKER_F_FIXED, but no one is.
Fix this by introduce a new agrument for create_io_worker() to indicate
if it is the first worker.

Fixes: 3d4e4fa ("io-wq: fix no lock protection of acct->nr_worker")
Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20210808135434.68667-3-haoxu@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Hao Xu

fs/io-wq.c

@@ -129,7 +129,7 @@ struct io_cb_cancel_data {
 	bool cancel_all;
 };
 
-static void create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index);
+static void create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index, bool first);
 static void io_wqe_dec_running(struct io_worker *worker);
 
 static bool io_worker_get(struct io_worker *worker)
@@ -248,18 +248,20 @@ static void io_wqe_wake_worker(struct io_wqe *wqe, struct io_wqe_acct *acct)
 	rcu_read_unlock();
 
 	if (!ret) {
-		bool do_create = false;
+		bool do_create = false, first = 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);
+			if (!acct->nr_workers)
+				first = true;
 			acct->nr_workers++;
 			do_create = true;
 		}
 		raw_spin_unlock_irq(&wqe->lock);
 		if (do_create)
-			create_io_worker(wqe->wq, wqe, acct->index);
+			create_io_worker(wqe->wq, wqe, acct->index, first);
 	}
 }
 
@@ -282,20 +284,22 @@ static void create_worker_cb(struct callback_head *cb)
 	struct io_wq *wq;
 	struct io_wqe *wqe;
 	struct io_wqe_acct *acct;
-	bool do_create = false;
+	bool do_create = false, first = false;
 
 	cwd = container_of(cb, struct create_worker_data, work);
 	wqe = cwd->wqe;
 	wq = wqe->wq;
 	acct = &wqe->acct[cwd->index];
 	raw_spin_lock_irq(&wqe->lock);
 	if (acct->nr_workers < acct->max_workers) {
+		if (!acct->nr_workers)
+			first = true;
 		acct->nr_workers++;
 		do_create = true;
 	}
 	raw_spin_unlock_irq(&wqe->lock);
 	if (do_create) {
-		create_io_worker(wq, cwd->wqe, cwd->index);
+		create_io_worker(wq, wqe, cwd->index, first);
 	} else {
 		atomic_dec(&acct->nr_running);
 		io_worker_ref_put(wq);
@@ -637,7 +641,7 @@ void io_wq_worker_sleeping(struct task_struct *tsk)
 	raw_spin_unlock_irq(&worker->wqe->lock);
 }
 
-static void create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index)
+static void create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index, bool first)
 {
 	struct io_wqe_acct *acct = &wqe->acct[index];
 	struct io_worker *worker;
@@ -678,7 +682,7 @@ 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 == 1) && (worker->flags & IO_WORKER_F_BOUND))
+	if (first && (worker->flags & IO_WORKER_F_BOUND))
 		worker->flags |= IO_WORKER_F_FIXED;
 	raw_spin_unlock_irq(&wqe->lock);
 	wake_up_new_task(tsk);