io_uring: abstract out provided buffer list selection

In preparation for providing another way to select a buffer, move the
existing logic into a helper.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3448,17 +3448,15 @@ static int io_buffer_add_list(struct io_ring_ctx *ctx,
 	return xa_err(xa_store(&ctx->io_bl_xa, bgid, bl, GFP_KERNEL));
 }
 
-static void __user *io_buffer_select(struct io_kiocb *req, size_t *len,
+static void __user *io_provided_buffer_select(struct io_kiocb *req, size_t *len,
+					      struct io_buffer_list *bl,
 					      unsigned int issue_flags)
 {
-	struct io_buffer *kbuf = req->kbuf;
-	struct io_ring_ctx *ctx = req->ctx;
-	struct io_buffer_list *bl;
+	struct io_buffer *kbuf;
 
-	io_ring_submit_lock(req->ctx, issue_flags);
+	if (list_empty(&bl->buf_list))
+		return ERR_PTR(-ENOBUFS);
 
-	bl = io_buffer_get_list(ctx, req->buf_index);
-	if (bl && !list_empty(&bl->buf_list)) {
 	kbuf = list_first_entry(&bl->buf_list, struct io_buffer, list);
 	list_del(&kbuf->list);
 	if (*len > kbuf->len)
@@ -3467,10 +3465,24 @@ static void __user *io_buffer_select(struct io_kiocb *req, size_t *len,
 	req->kbuf = kbuf;
 	io_ring_submit_unlock(req->ctx, issue_flags);
 	return u64_to_user_ptr(kbuf->addr);
-	}
+}
+
+static void __user *io_buffer_select(struct io_kiocb *req, size_t *len,
+				     unsigned int issue_flags)
+{
+	struct io_ring_ctx *ctx = req->ctx;
+	struct io_buffer_list *bl;
 
+	io_ring_submit_lock(req->ctx, issue_flags);
+
+	bl = io_buffer_get_list(ctx, req->buf_index);
+	if (unlikely(!bl)) {
 		io_ring_submit_unlock(req->ctx, issue_flags);
 		return ERR_PTR(-ENOBUFS);
+	}
+
+	/* selection helpers drop the submit lock again, if needed */
+	return io_provided_buffer_select(req, len, bl, issue_flags);
 }
 
 #ifdef CONFIG_COMPAT