This currently isn't supported, and due to a recent commit, it also
cannot easily be supported by io_uring due to hash_node and IOPOLL
completion data overlapping.

This can be revisited if we ever do support cancelations of requests
that have gone to the block stack.

Suggested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
index 197474911f04..ee7b49f47cb5 100644
--- a/io_uring/uring_cmd.c
+++ b/io_uring/uring_cmd.c
@@ -104,6 +104,15 @@ void io_uring_cmd_mark_cancelable(struct io_uring_cmd *cmd,
 	struct io_kiocb *req = cmd_to_io_kiocb(cmd);
 	struct io_ring_ctx *ctx = req->ctx;
 
+	/*
+	 * Doing cancelations on IOPOLL requests are not supported. Both
+	 * because they can't get canceled in the block stack, but also
+	 * because iopoll completion data overlaps with the hash_node used
+	 * for tracking.
+	 */
+	if (ctx->flags & IORING_SETUP_IOPOLL)
+		return;
+
 	if (!(cmd->flags & IORING_URING_CMD_CANCELABLE)) {
 		cmd->flags |= IORING_URING_CMD_CANCELABLE;
 		io_ring_submit_lock(ctx, issue_flags);
-- 
Jens Axboe