io_uring: set task_work notify method at init time

While doing so, switch SQPOLL to TWA_SIGNAL_NO_IPI as well, as that
just does a task wakeup and then we can remove the special wakeup we
have in task_work_add.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20220426014904.60384-5-axboe@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -366,6 +366,7 @@ struct io_ring_ctx {
 
 		struct io_rings		*rings;
 		unsigned int		flags;
+		enum task_work_notify_mode	notify_method;
 		unsigned int		compat: 1;
 		unsigned int		drain_next: 1;
 		unsigned int		restricted: 1;
@@ -2621,8 +2622,8 @@ static void tctx_task_work(struct callback_head *cb)
 static void io_req_task_work_add(struct io_kiocb *req, bool priority)
 {
 	struct task_struct *tsk = req->task;
+	struct io_ring_ctx *ctx = req->ctx;
 	struct io_uring_task *tctx = tsk->io_uring;
-	enum task_work_notify_mode notify;
 	struct io_wq_work_node *node;
 	unsigned long flags;
 	bool running;
@@ -2645,18 +2646,8 @@ static void io_req_task_work_add(struct io_kiocb *req, bool priority)
 	if (running)
 		return;
 
-	/*
-	 * SQPOLL kernel thread doesn't need notification, just a wakeup. For
-	 * all other cases, use TWA_SIGNAL unconditionally to ensure we're
-	 * processing task_work. There's no reliable way to tell if TWA_RESUME
-	 * will do the job.
-	 */
-	notify = (req->ctx->flags & IORING_SETUP_SQPOLL) ? TWA_NONE : TWA_SIGNAL;
-	if (likely(!task_work_add(tsk, &tctx->task_work, notify))) {
-		if (notify == TWA_NONE)
-			wake_up_process(tsk);
+	if (likely(!task_work_add(tsk, &tctx->task_work, ctx->notify_method)))
 		return;
-	}
 
 	spin_lock_irqsave(&tctx->task_lock, flags);
 	tctx->task_running = false;
@@ -11335,6 +11326,14 @@ static __cold int io_uring_create(unsigned entries, struct io_uring_params *p,
 	if (!capable(CAP_IPC_LOCK))
 		ctx->user = get_uid(current_user());
 
+	/*
+	 * For SQPOLL, we just need a wakeup, always.
+	 */
+	if (ctx->flags & IORING_SETUP_SQPOLL)
+		ctx->notify_method = TWA_SIGNAL_NO_IPI;
+	else
+		ctx->notify_method = TWA_SIGNAL;
+
 	/*
 	 * This is just grabbed for accounting purposes. When a process exits,
 	 * the mm is exited and dropped before the files, hence we need to hang