The mad_client will be initialized in enable_device_and_get(), while the
devices_rwsem will be downgraded to a read semaphore. There is a window
that leads to the failed initialization for cm_client, since it can not
get matched mad port from ib_mad_port_list, and the matched mad port will
be added to the list after that.

    mad_client    |                       cm_client
------------------|--------------------------------------------------------
ib_register_device|
enable_device_and_get
down_write(&devices_rwsem)
xa_set_mark(&devices, DEVICE_REGISTERED)
downgrade_write(&devices_rwsem)
                  |
                  |ib_cm_init
                  |ib_register_client(&cm_client)
                  |down_read(&devices_rwsem)
                  |xa_for_each_marked (&devices, DEVICE_REGISTERED)
                  |add_client_context
                  |cm_add_one
                  |ib_register_mad_agent
                  |ib_get_mad_port
                  |__ib_get_mad_port
                  |list_for_each_entry(entry, &ib_mad_port_list, port_list)
                  |return NULL
                  |up_read(&devices_rwsem)
                  |
add_client_context|
ib_mad_init_device|
ib_mad_port_open  |
list_add_tail(&port_priv->port_list, &ib_mad_port_list)
up_read(&devices_rwsem)
                  |

Fix it by using down_write(&devices_rwsem) in ib_register_client().

Fixes: d0899892 ("RDMA/device: Provide APIs from the core code to help unregistration")
Link: https://lore.kernel.org/r/20240203035313.98991-1-lishifeng@sangfor.com.cnSuggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Shifeng Li <lishifeng@sangfor.com.cn>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>