ublk: make sure io cmd handled in submitter task context

In well-done ublk server implementation, ublk io command won't be
linked into any link chain. Meantime they are always handled in no-wait
style, so basically io cmd is always handled in submitter task context.

However, the server may set IOSQE_ASYNC, or io command is linked to one
chain mistakenly, then we may still run into io-wq context and
ctx->uring_lock isn't held.

So in case of IO_URING_F_UNLOCKED, schedule this command by
io_uring_cmd_complete_in_task to force running it in submitter task. Then
ublk_ch_uring_cmd_local() is guaranteed to run with context uring_lock held,
and we needn't to worry about sync among submission code path any more.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20231009093324.957829-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -1811,7 +1811,8 @@ static inline struct request *__ublk_check_and_get_req(struct ublk_device *ub,
 	return NULL;
 }
 
-static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
+static inline int ublk_ch_uring_cmd_local(struct io_uring_cmd *cmd,
+		unsigned int issue_flags)
 {
 	/*
 	 * Not necessary for async retry, but let's keep it simple and always
@@ -1825,9 +1826,28 @@ static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
 		.addr = READ_ONCE(ub_src->addr)
 	};
 
+	WARN_ON_ONCE(issue_flags & IO_URING_F_UNLOCKED);
+
 	return __ublk_ch_uring_cmd(cmd, issue_flags, &ub_cmd);
 }
 
+static void ublk_ch_uring_cmd_cb(struct io_uring_cmd *cmd,
+		unsigned int issue_flags)
+{
+	ublk_ch_uring_cmd_local(cmd, issue_flags);
+}
+
+static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
+{
+	/* well-implemented server won't run into unlocked */
+	if (unlikely(issue_flags & IO_URING_F_UNLOCKED)) {
+		io_uring_cmd_complete_in_task(cmd, ublk_ch_uring_cmd_cb);
+		return -EIOCBQUEUED;
+	}
+
+	return ublk_ch_uring_cmd_local(cmd, issue_flags);
+}
+
 static inline bool ublk_check_ubuf_dir(const struct request *req,
 		int ubuf_dir)
 {