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Subject: Re: NFSD Regression: client observing a file while other client
 writes to it leads to stale local cache
To:     "J. Bruce Fields" <bfields@fieldses.org>
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
References: <c3efd7e8-dc08-ac1f-9bee-7a7b0d8ac5fb@rothenpieler.org>
 <20210301184633.GA14881@fieldses.org>
 <86ef3b71-bcb4-767c-40a0-461d082f5d44@rothenpieler.org>
 <20210302010629.GB16303@fieldses.org>
From:   Timo Rothenpieler <timo@rothenpieler.org>
Message-ID: <2d4e7965-903f-2319-d2d2-65116f50400c@rothenpieler.org>
Date:   Tue, 2 Mar 2021 02:23:05 +0100
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On 02.03.2021 02:06, J. Bruce Fields wrote:
> On Tue, Mar 02, 2021 at 02:03:58AM +0100, Timo Rothenpieler wrote:
>> On 01.03.2021 19:46, J. Bruce Fields wrote:
>>> Thanks for the bisecting this and reporting the results.
>>>
>>> The behavior you describe is probably not a bug: NFS close-to-open
>>> caching semantics don't guarantee you'll see updates on the reading
>>> client in this case.
>>>
>>> Nevertheless, I don't understand why the behavior changed with that
>>> commit, and I'd like to.
>>>
>>> The only change in behavior I'd expect would be that the writing clie=
nt
>>> would be granted a read delegation.  But I wouldn't expect that to
>>> change how it writes back data, or how the reading client checks for
>>
>> The writing side of things does not seem to be affected.
>> At any point during testing, the file on the servers filesystem is
>> in the state I'd expect it to be, growing by one line per second.
>>
>> What's also a bit odd about this is that after both clients have
>> closed the file, the reading client still sees an older version of
>> the file, the one state it was in when it was first opened on the
>> client.
>> The file size in stat()/ls -l mismatches between the reading client
>> and the server.
>> It stays in that state for some seemingly arbitrary amount of time.
>> Observing the file in any way(cat/tail, or even just ls) appears to
>> extend the wait time.
>> After leaving the file alone for long enough on the reading client,
>> it snaps back to reality.
>=20
> Hm, I wonder if we're (incorrectly) giving out a delegation to the
> reading client.  Is this 100% reproduceable?

Server and reading client disagreeing about the files size and contents=20
is 100% reproducible, even after the writing client has long and=20
definitely closed the file.

I'm not 100% certain about the exact timing and extension of timing,=20
since I never did any intensive testing on that. But it stays mismatched =

between client and server like that for at least several minutes.


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