Re: [syzbot] [nfs?] INFO: task hung in nfsd_nl_listener_set_doit

Chuck Lever
2024-09-04T14:23:55Z

On Mon, Sep 02, 2024 at 11:57:55AM +1000, NeilBrown wrote:
> On Sun, 01 Sep 2024, syzbot wrote:
> > syzbot has found a reproducer for the following issue on:
>
> I had a poke around using the provided disk image and kernel for
> exploring.
>
> I think the problem is demonstrated by this stack :
>
> [<0>] rpc_wait_bit_killable+0x1b/0x160
> [<0>] __rpc_execute+0x723/0x1460
> [<0>] rpc_execute+0x1ec/0x3f0
> [<0>] rpc_run_task+0x562/0x6c0
> [<0>] rpc_call_sync+0x197/0x2e0
> [<0>] rpcb_register+0x36b/0x670
> [<0>] svc_unregister+0x208/0x730
> [<0>] svc_bind+0x1bb/0x1e0
> [<0>] nfsd_create_serv+0x3f0/0x760
> [<0>] nfsd_nl_listener_set_doit+0x135/0x1a90
> [<0>] genl_rcv_msg+0xb16/0xec0
> [<0>] netlink_rcv_skb+0x1e5/0x430
>
> No rpcbind is running on this host so that "svc_unregister" takes a
> long time. Maybe not forever but if a few of these get queued up all
> blocking some other thread, then maybe that pushed it over the limit.
>
> The fact that rpcbind is not running might not be relevant as the test
> messes up the network. "ping 127.0.0.1" stops working.
>
> So this bug comes down to "we try to contact rpcbind while holding a
> mutex and if that gets no response and no error, then we can hold the
> mutex for a long time".
>
> Are we surprised? Do we want to fix this? Any suggestions how?

In the past, we've tried to address "hanging upcall" issues where
the kernel part of an administrative command needs a user space
service that isn't working or present. (eg mount needing a running
gssd)

If NFSD is using the kernel RPC client for the upcall, then maybe
adding the RPC_TASK_SOFTCONN flag might turn the hang into an
immediate failure.

IMO this should be addressed.

--
Chuck Lever