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

[Li Lingfeng]

在 2024/11/26 1:32, Mark Liam Brown 写道:
> On Mon, Nov 25, 2024 at 1:48 PM Li Lingfeng <lilingfeng3@xxxxxxxxxx> wrote:
> > 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 disagree with that comment. Most small companies have NFS client and
> NFS server on the same machine, the client being used to allow logins
> by users, or to support schroot or containers.
>
> Mark

Sorry for my hasty conclusion.

By the way, nfsd_reply_cache_shrinker triggers this too.

Li