io_uring: close lookup gap for dependent next work

When we find new work to process within the work handler, we queue the
linked timeout before we have issued the new work. This can be
problematic for very short timeouts, as we have a window where the new
work isn't visible.

Allow the work handler to store a callback function for this in the work
item, and flag it with IO_WQ_WORK_CB if the caller has done so. If that
is set, then io-wq will call the callback when it has setup the new work
item.
Reported-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -427,6 +427,9 @@ static void io_worker_handle_work(struct io_worker *worker)
 		worker->cur_work = work;
 		spin_unlock_irq(&worker->lock);
 
+		if (work->flags & IO_WQ_WORK_CB)
+			work->func(&work);
+
 		if ((work->flags & IO_WQ_WORK_NEEDS_FILES) &&
 		    current->files != work->files) {
 			task_lock(current);

fs/io-wq.h

@@ -11,6 +11,7 @@ enum {
 	IO_WQ_WORK_NEEDS_FILES	= 16,
 	IO_WQ_WORK_UNBOUND	= 32,
 	IO_WQ_WORK_INTERNAL	= 64,
+	IO_WQ_WORK_CB		= 128,
 
 	IO_WQ_HASH_SHIFT	= 24,	/* upper 8 bits are used for hash key */
 };
@@ -22,7 +23,10 @@ enum io_wq_cancel {
 };
 
 struct io_wq_work {
-	struct list_head list;
+	union {
+		struct list_head list;
+		void *data;
+	};
 	void (*func)(struct io_wq_work **);
 	unsigned flags;
 	struct files_struct *files;

fs/io_uring.c

@@ -2679,6 +2679,15 @@ static int __io_submit_sqe(struct io_kiocb *req, struct io_kiocb **nxt,
 	return 0;
 }
 
+static void io_link_work_cb(struct io_wq_work **workptr)
+{
+	struct io_wq_work *work = *workptr;
+	struct io_kiocb *link = work->data;
+
+	io_queue_linked_timeout(link);
+	work->func = io_wq_submit_work;
+}
+
 static void io_wq_submit_work(struct io_wq_work **workptr)
 {
 	struct io_wq_work *work = *workptr;
@@ -2725,8 +2734,11 @@ static void io_wq_submit_work(struct io_wq_work **workptr)
 
 		io_prep_async_work(nxt, &link);
 		*workptr = &nxt->work;
-		if (link)
-			io_queue_linked_timeout(link);
+		if (link) {
+			nxt->work.flags |= IO_WQ_WORK_CB;
+			nxt->work.func = io_link_work_cb;
+			nxt->work.data = link;
+		}
 	}
 }