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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>
 <2d4e7965-903f-2319-d2d2-65116f50400c@rothenpieler.org>
 <20210304144228.GD17512@fieldses.org> <20210305223141.GD3813@fieldses.org>
From:   Timo Rothenpieler <timo@rothenpieler.org>
Message-ID: <db00d956-ea4d-2cfb-a5d6-a9498e3a4163@rothenpieler.org>
Date:   Sat, 6 Mar 2021 01:01:53 +0100
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On 05.03.2021 23:31, J. Bruce Fields wrote:
> Yeah, it's completely failing to provide close-to-open cache consistenc=
y
> there in most cases.  Arghh.
>=20
> I have some idea how to fix it, but for now it should just be reverted.=

> I'll post that pending some testing.
>=20
> I've also added a pynfs test for this case (4.1 test DELEG9).  We shoul=
d
> probably have some more.
>=20
> So, thanks for the report.
>=20
> I'd still be a little cautious about your use--NFS can't give you a lot=

> of guarantees when you've got a file open for read and write
> simultaneously.  The writing client can delay those writes at least
> until the writing application closes, and the reading isn't necessarily=

> going to revalidate its cache until it the reading application closes
> and reopens.  And weirder behavior is possible: for exapmle when the
> writer does write back, I don't think there's any guarantee it will
> write in order, so you could end up temporarily with NULLs in the
> middle.

As long as it doesn't get stuck for extended amounts of time, it being a =

bit janked is fine.
A close and re-open bringing it back up to date that's all I can ask for.=


Not sure how "tail -f" operates internally. It does seem to have special =

handling for remote files/network filesystems though, so it might be=20
doing the right thing already.
But as long as calling "cat" on the file repeatedly works, that's fine.


> But that particular regression was my fault, thanks again for bisecting=

> and reporting.
>=20

I've been running a 5.10 Kernel with the commit reverted for a the whole =

week now, and haven't run into any issues so far.


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