[bug report] deploying both NFS client and server on the same machine triggle hungtask

[Li Lingfeng <lilingfeng3@huawei.com>]

Hi, we have found a hungtask issue recently.

Commit 7746b32f467b ("NFSD: add shrinker to reap courtesy clients on low
memory condition") adds a shrinker to NFSD, which causes NFSD to try to
obtain shrinker_rwsem when starting and stopping services.

Deploying both NFS client and server on the same machine may lead to the
following issue, since they will share the global shrinker_rwsem.

     nfsd                            nfs
                             drop_cache // hold shrinker_rwsem
                             write back, wait for rpc_task to exit
// stop nfsd threads
svc_set_num_threads
// clean up xprts
svc_xprt_destroy_all
                             rpc_check_timeout
                              rpc_check_connected
                              // wait for the connection to be disconnected
unregister_shrinker
// wait for shrinker_rwsem

Normally, the client's rpc_task will exit after the server's nfsd thread
has processed the request.
When all the server's nfsd threads exit, the client’s rpc_task is expected
to detect the network connection being disconnected and exit.
However, although the server has executed svc_xprt_destroy_all before
waiting for shrinker_rwsem, the network connection is not actually
disconnected. Instead, the operation to close the socket is simply added
to the task_works queue.

svc_xprt_destroy_all
  ...
  svc_sock_free
   sockfd_put
    fput_many
     init_task_work // ____fput
     task_work_add // add to task->task_works

The actual disconnection of the network connection will only occur after
the current process finishes.
do_exit
  exit_task_work
   task_work_run
   ...
    ____fput // close sock

Although it is not a common practice to deploy NFS client and server on
the same machine, I think this issue still needs to be addressed,
otherwise it will cause all processes trying to acquire the shrinker_rwsem
to hang.

I don't have any ideas yet on how to solve this problem, does anyone have
any suggestions?

Thanks.