[PATCH V3 1/6] ublk: allow io buffer register/unregister command issued from other task contexts

[Ming Lei <ming.lei@redhat.com>]

`ublk_queue` is read only for io buffer register/unregister command. Both
`ublk_io` and block layer request are read-only for IO buffer register/
unregister command.

So the two command can be issued from other task contexts.

Not same with other three ublk commands, these two are for handling target
IO only, we shouldn't limit their issue task context, otherwise it becomes
hard for ublk server(backend) to use zero copy feature.

Reported-by: Uday Shankar <ushankar@purestorage.com>
Closes: https://lore.kernel.org/linux-block/20250410-ublk_task_per_io-v3-2-b811e8f4554a@purestorage.com/
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/ublk_drv.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index cb612151e9a1..31f06e734250 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -2057,6 +2057,12 @@ static bool ublk_get_data(const struct ublk_queue *ubq, struct ublk_io *io)
 	return ublk_start_io(ubq, req, io);
 }
 
+static bool is_io_buf_reg_unreg_cmd(unsigned int cmd_op)
+{
+	return _IOC_NR(cmd_op) == UBLK_IO_REGISTER_IO_BUF ||
+		_IOC_NR(cmd_op) == UBLK_IO_UNREGISTER_IO_BUF;
+}
+
 static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
 			       unsigned int issue_flags,
 			       const struct ublksrv_io_cmd *ub_cmd)
@@ -2076,8 +2082,15 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
 		goto out;
 
 	ubq = ublk_get_queue(ub, ub_cmd->q_id);
-	if (ubq->ubq_daemon && ubq->ubq_daemon != current)
-		goto out;
+	/*
+	 * Both `ublk_io` and block layer request are read-only for IO
+	 * buffer register/unregister command, so the two are allowed to be
+	 * issued from other task contexts
+	 */
+	if (!is_io_buf_reg_unreg_cmd(cmd_op)) {
+		if (ubq->ubq_daemon && ubq->ubq_daemon != current)
+			goto out;
+	}
 
 	if (tag >= ubq->q_depth)
 		goto out;
-- 
2.47.0