io-wq: fork worker threads from original task

Instead of using regular kthread kernel threads, create kernel threads
that are like a real thread that the task would create. This ensures that
we get all the context that we need, without having to carry that state
around. This greatly reduces the code complexity, and the risk of missing
state for a given request type.

With the move away from kthread, we can also dump everything related to
assigned state to the new threads.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.h

@@ -137,6 +137,7 @@ static inline void io_wq_worker_running(struct task_struct *tsk)
 
 static inline bool io_wq_current_is_worker(void)
 {
-	return in_task() && (current->flags & PF_IO_WORKER);
+	return in_task() && (current->flags & PF_IO_WORKER) &&
+		current->pf_io_worker;
 }
 #endif

fs/io_uring.c

@@ -1633,6 +1633,9 @@ static struct io_kiocb *__io_queue_async_work(struct io_kiocb *req)
 	struct io_kiocb *link = io_prep_linked_timeout(req);
 	struct io_uring_task *tctx = req->task->io_uring;
 
+	BUG_ON(!tctx);
+	BUG_ON(!tctx->io_wq);
+
 	trace_io_uring_queue_async_work(ctx, io_wq_is_hashed(&req->work), req,
 					&req->work, req->flags);
 	io_wq_enqueue(tctx->io_wq, &req->work);
@@ -9240,6 +9243,10 @@ static int io_uring_flush(struct file *file, void *data)
 	struct io_uring_task *tctx = current->io_uring;
 	struct io_ring_ctx *ctx = file->private_data;
 
+	/* Ignore helper thread files exit */
+	if (current->flags & PF_IO_WORKER)
+		return 0;
+
 	if (fatal_signal_pending(current) || (current->flags & PF_EXITING)) {
 		io_uring_cancel_task_requests(ctx, NULL);
 		io_req_caches_free(ctx, current);

include/linux/sched.h

@@ -895,6 +895,9 @@ struct task_struct {
 	/* CLONE_CHILD_CLEARTID: */
 	int __user			*clear_child_tid;
 
+	/* PF_IO_WORKER */
+	void				*pf_io_worker;
+
 	u64				utime;
 	u64				stime;
 #ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME