io_uring: disallow defer-tw run w/ no submitters

We try to restrict CQ waiters when IORING_SETUP_DEFER_TASKRUN is set,
but if nothing has been submitted yet it'll allow any waiter, which
violates the contract.

Fixes: c0e0d6ba ("io_uring: add IORING_SETUP_DEFER_TASKRUN")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/b4f0d3f14236d7059d08c5abe2661ef0b78b5528.1662652536.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -1169,13 +1169,8 @@ int __io_run_local_work(struct io_ring_ctx *ctx, bool locked)
 	int ret;
 	unsigned int loops = 1;
 
-	if (unlikely(ctx->submitter_task != current)) {
-		/* maybe this is before any submissions */
-		if (!ctx->submitter_task)
-			return 0;
-
+	if (unlikely(ctx->submitter_task != current))
 		return -EEXIST;
-	}
 
 	node = io_llist_xchg(&ctx->work_llist, &fake);
 	ret = 0;

io_uring/io_uring.h

@@ -331,13 +331,8 @@ static inline struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx)
 
 static inline bool io_allowed_run_tw(struct io_ring_ctx *ctx)
 {
-	if (!(ctx->flags & IORING_SETUP_DEFER_TASKRUN))
-		return true;
-	if (unlikely(ctx->submitter_task != current)) {
-		/* maybe this is before any submissions */
-		return !ctx->submitter_task;
-	}
-	return true;
+	return likely(!(ctx->flags & IORING_SETUP_DEFER_TASKRUN) ||
+		      ctx->submitter_task == current);
 }
 
 #endif