io_uring: don't touch sqd->thread off tw add

With IORING_SETUP_SQPOLL all requests are created by the SQPOLL task,
which means that req->task should always match sqd->thread. Since
accesses to sqd->thread should be separately protected, use req->task
in io_req_normal_work_add() instead.

Note, in the eyes of io_req_normal_work_add(), the SQPOLL task struct
is always pinned and alive, and sqd->thread can either be the task or
NULL. It's only problematic if the compiler decides to reload the value
after the null check, which is not so likely.

Cc: [email protected]
Cc: Bui Quang Minh <[email protected]>
Reported-by: lizetao <[email protected]>
Fixes: 78f9b61 ("io_uring: wake SQPOLL task when task_work is added to an empty queue")
Signed-off-by: Pavel Begunkov <[email protected]>
Link: https://lore.kernel.org/r/1cbbe72cf32c45a8fee96026463024cd8564a7d7.1736541357.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <[email protected]>

io_uring/io_uring.c

@@ -1226,10 +1226,7 @@ static void io_req_normal_work_add(struct io_kiocb *req)
 
 	/* SQPOLL doesn't need the task_work added, it'll run it itself */
 	if (ctx->flags & IORING_SETUP_SQPOLL) {
-		struct io_sq_data *sqd = ctx->sq_data;
-
-		if (sqd->thread)
-			__set_notify_signal(sqd->thread);
+		__set_notify_signal(tctx->task);
 		return;
 	}