New subject: Client id expiration - multiple clients using same hostname and deadlock issues.

Tuesday, 25 April 2023

Please open a github issue for this. We definitely need to not crash or
deadlock.

It sounds like as long as we don't fail miserably, the client can't expect
anything.

I'm not sure if we can handle the situation more gracefully and still do the
right thing in cases where this situation is caused by a client whose IP
address has changed for some reason or other things that might make an
actual single client appear to the server as multiple clients.

Frank

From: Deepak Arumugam Sankara Subramanian
[mailto:deepakarumugam.s@nutanix.com] 
Sent: Tuesday, April 25, 2023 2:04 PM
To: devel(a)lists.nfs-ganesha.org
Cc: Pradeep Thomas <pradeep.thomas(a)nutanix.com>; Pragyashree Gogoi
<pragyashree.gogoi(a)nutanix.com>; Sagar Singh <sagar.singh(a)nutanix.com>;
Gaurav Gangalwar <gaurav.gangalwar(a)nutanix.com>; Andrew Lin
<andrew.lin(a)nutanix.com&gt;
Subject: [NFS-Ganesha-Devel] Client id expiration - multiple clients using
same hostname and deadlock issues.

Hi ,

We recently ran into some issues in client-id expiration code paths. 

We've had multiple issues where a CREATE_SESSION tries to expire a clientid
when the clientid is being used by other rpcs like OPEN, EXCHANGE_ID etc.You
typically don't expect a client to send a OPEN/EXCHANGE_ID rpc while the
server is processing a CREATE_SESSSION rpc unless the client is misbehaving.
We found that in our labs/test-beds multiple clients had the same hostname
set and ganesha was mapping them to the same client record.

Now although the clients were misbehaving we feel that the server should
handle/fail gracefully. In these issues the server usually runs into an
unexpected assert,segfault and crashes or it runs into a deadlock and hangs
forever .

Q1: We need some recommendations on whats the right thing to do when
multiple clients use the same owner_id.

The RFC says this about co_ownerid 

The string should be unique so that multiple clients do not
      present the same string.  The consequences of two clients
      presenting the same string range from one client getting an error
      to one client having its leased state abruptly and unexpectedly
      cancelled.

but we wanted to know if one of these responses is better than other. 

These are some more details on the individual issues raised from setups
having the same hostname

Deadlock:

Two client ids say c1 and c2 from 2 different clients were associated with
the same co_ownerid and same record cr1

1.	Client 1 -> thread 1 was doing a open. It was inside the
get_state_owner function trying to get a owner for the open state. It
acquired ht_owner->partitions[15].lock created a new open owner and was
trying to hold the mutex on cr1 aka nfs4_owner->so_clientrec->cid_mutex so
it could add the owner to the client record cr1.

2.	Client2 -> thread 2 was doing a create session. It was inside
nfs_clientid_expire function. It was parsing the open owner list inside c1
and trying to 'delete' each owner (the second while(true) loop). It was
holding nfs4_owner->so_clientrec->cid_mutex and trying to get
ht_owner->partitions[15].lock 

Q2: If we were to fix this deadlock, whats the recommended order for
acquiring the locks - should we acquire the client_rec->cid_mutex first and
then acquire ht_owner table lock or vice versa This looks like it could be a
common deadlock pattern we might've encountered before since we have 2
structures referencing each other each with their own locks

I can update more details on the other failures(asserts, segfaults etc). Let
me know if that is needed

Thanks,
Deepak

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Re: Client id expiration - multiple clients using same hostname and deadlock issues.