Re: NFSD threads hang when destroying a session or client ID

Chuck Lever
2025-01-21T19:43:38Z

On 1/21/25 2:38 PM, Tom Talpey wrote:
> On 1/21/2025 12:35 PM, Jeff Layton via Bugspray Bot wrote:
>> Jeff Layton writes via Kernel.org Bugzilla:
>>
>> (In reply to Chuck Lever from comment #7)
>>> The trace captures I've reviewed suggest that a callback session is
>>> in use,
>>> so I would say the NFS minor version is 1 or higher. Perhaps it's not
>>> the
>>> RPC_SIGNALLED test above that is the problem, but the one later in
>>> nfsd4_cb_sequence_done().
>>
>>
>> Ok, good. Knowing that it's not v4.0 allows us to rule out some
>> codepaths.
>> There are a couple of other cases where we goto need_restart:
>>
>> The NFS4ERR_BADSESSION case does this, and also if it doesn't get a
>> reply at all (case 1).
>
> Note that one thread in Benoît's recent logs is stuck in
> nfsd4_bind_conn_to_session(), and three threads also in
> nfsd4_destroy_session(), so there is certainly some
> session/connection dance going on. Combining an invalid
> replay cache entry could easily make things worse.

Yes, the client returns RETRY_UNCACHED_REP for some of the backchannel
operations. NFSD never asserts cachethis in CB_SEQUENCE. I'm trying to
understand why NFSD would skip incrementing its slot sequence number.

> There's also one thread in nfsd4_destroy_clientid(), which
> seems important, but odd. And finally, the laundromat is
> running. No shortage of races!

The hangs are all related here: they are waiting for flush_workqueue()
on the callback workqueue. In v6.1, there is only one callback_wq and
it's max_active is 1. If the current work item hangs, then that work
queue stalls.

> Tom.
>
>
>> There is also this that looks a little sketchy:
>>
>> ------------8<-------------------
>>          trace_nfsd_cb_free_slot(task, cb);
>>          nfsd41_cb_release_slot(cb);
>>
>>          if (RPC_SIGNALLED(task))
>>                  goto need_restart;
>> out:
>>          return ret;
>> retry_nowait:
>>          if (rpc_restart_call_prepare(task))
>>                  ret = false;
>>          goto out;
>> need_restart:
>>          if (!test_bit(NFSD4_CLIENT_CB_KILL, &clp->cl_flags)) {
>>                  trace_nfsd_cb_restart(clp, cb);
>>                  task->tk_status = 0;
>>                  cb->cb_need_restart = true;
>>          }
>>          return false;
>> ------------8<-------------------
>>
>> Probably now the same bug, but it looks like if RPC_SIGNALLED returns
>> true, then it'll restart the RPC after releasing the slot. It seems
>> like that could break the reply cache handling, as the restarted call
>> could be on a different slot. I'll look at patching that, at least,
>> though I'm not sure it's related to the hang.
>>
>> More notes. The only way RPC_TASK_SIGNALLED gets set is:
>>
>>     nfsd4_process_cb_update()
>>        rpc_shutdown_client()
>>            rpc_killall_tasks()
>>
>> That gets called if:
>>
>>          if (clp->cl_flags & NFSD4_CLIENT_CB_FLAG_MASK)
>>                  nfsd4_process_cb_update(cb);
>>
>> Which means that NFSD4_CLIENT_CB_UPDATE was probably set?
>> NFSD4_CLIENT_CB_KILL seems less likely since that would nerf the
>> cb_need_restart handling.
>>
>> View: https://bugzilla.kernel.org/show_bug.cgi?id=219710#c10
>> You can reply to this message to join the discussion.
>

--
Chuck Lever