io_uring: register single issuer task at creation

Instead of picking the task from the first submitter task, rather use the
creator task or in the case of disabled (IORING_SETUP_R_DISABLED) the
enabling task.

This approach allows a lot of simplification of the logic here. This
removes init logic from the submission path, which can always be a bit
confusing, but also removes the need for locking to write (or read) the
submitter_task.

Users that want to move a ring before submitting can create the ring
disabled and then enable it on the submitting task.
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Fixes: 97bbdc06 ("io_uring: add IORING_SETUP_SINGLE_ISSUER")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -3357,6 +3357,10 @@ static __cold int io_uring_create(unsigned entries, struct io_uring_params *p,
 		goto err;
 	}
 
+	if (ctx->flags & IORING_SETUP_SINGLE_ISSUER
+	    && !(ctx->flags & IORING_SETUP_R_DISABLED))
+		ctx->submitter_task = get_task_struct(current);
+
 	file = io_uring_get_file(ctx);
 	if (IS_ERR(file)) {
 		ret = PTR_ERR(file);
@@ -3548,6 +3552,9 @@ static int io_register_enable_rings(struct io_ring_ctx *ctx)
 	if (!(ctx->flags & IORING_SETUP_R_DISABLED))
 		return -EBADFD;
 
+	if (ctx->flags & IORING_SETUP_SINGLE_ISSUER && !ctx->submitter_task)
+		ctx->submitter_task = get_task_struct(current);
+
 	if (ctx->restrictions.registered)
 		ctx->restricted = 1;