Re: [PATCH] NFSv4: fix stateid refreshing when CLOSE racing with OPEN

[Chuck Lever <chuck.lever@oracle.com>]

> On Sep 4, 2020, at 6:55 AM, Benjamin Coddington <bcodding@redhat.com> wrote:
> 
> On 3 Sep 2020, at 23:04, Murphy Zhou wrote:
> 
>> Hi Benjamin,
>> 
>> On Thu, Sep 03, 2020 at 01:54:26PM -0400, Benjamin Coddington wrote:
>>> 
>>> On 11 Oct 2019, at 10:14, Trond Myklebust wrote:
>>>> On Fri, 2019-10-11 at 16:49 +0800, Murphy Zhou wrote:
>>>>> On Thu, Oct 10, 2019 at 02:46:40PM +0000, Trond Myklebust wrote:
>>>>>> On Thu, 2019-10-10 at 15:40 +0800, Murphy Zhou wrote:
>>> ...
>>>>>>> @@ -3367,14 +3368,16 @@ static bool
>>>>>>> nfs4_refresh_open_old_stateid(nfs4_stateid *dst,
>>>>>>> 			break;
>>>>>>> 		}
>>>>>>> 		seqid_open = state->open_stateid.seqid;
>>>>>>> -		if (read_seqretry(&state->seqlock, seq))
>>>>>>> -			continue;
>>>>>>> 
>>>>>>> 		dst_seqid = be32_to_cpu(dst->seqid);
>>>>>>> -		if ((s32)(dst_seqid - be32_to_cpu(seqid_open)) >= 0)
>>>>>>> +		if ((s32)(dst_seqid - be32_to_cpu(seqid_open)) > 0)
>>>>>>> 			dst->seqid = cpu_to_be32(dst_seqid + 1);
>>>>>> 
>>>>>> This negates the whole intention of the patch you reference in the
>>>>>> 'Fixes:', which was to allow us to CLOSE files even if seqid bumps
>>>>>> have
>>>>>> been lost due to interrupted RPC calls e.g. when using 'soft' or
>>>>>> 'softerr' mounts.
>>>>>> With the above change, the check could just be tossed out
>>>>>> altogether,
>>>>>> because dst_seqid will never become larger than seqid_open.
>>>>> 
>>>>> Hmm.. I got it wrong. Thanks for the explanation.
>>>> 
>>>> So to be clear: I'm not saying that what you describe is not a problem.
>>>> I'm just saying that the fix you propose is really no better than
>>>> reverting the entire patch. I'd prefer not to do that, and would rather
>>>> see us look for ways to fix both problems, but if we can't find such as
>>>> fix then that would be the better solution.
>>> 
>>> Hi Trond and Murphy Zhou,
>>> 
>>> Sorry to resurrect this old thread, but I'm wondering if any progress was
>>> made on this front.
>> 
>> This failure stoped showing up since v5.6-rc1 release cycle
>> in my records. Can you reproduce this on latest upstream kernel?
> 
> I'm seeing it on generic/168 on a v5.8 client against a v5.3 knfsd server.
> When I test against v5.8 server, the test takes longer to complete and I
> have yet to reproduce the livelock.
> 
> - on v5.3 server takes ~50 iterations to produce, each test completes in ~40
> seconds
> - on v5.8 server my test has run ~750 iterations without getting into
> the lock, each test takes ~60 seconds.
> 
> I suspect recent changes to the server have changed the timing of open
> replies such that the problem isn't reproduced on the client.

The Linux NFS server in v5.4 does behave differently than earlier
kernels with NFSv4.0, and it is performance-related. The filecache
went into v5.4, and that seems to change the frequency at which
the server offers delegations.

I'm looking into it, and learning a bunch.


--
Chuck Lever