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Hello,

[ please CC me any replies ]

While debugging some problems with exporting tmpfs volumes, I was
getting messages from the kernel similar to these:

/tmp/remote: inaccurate wcc data (mtime) detected, disabling wcc
(or in other cases, ctime)

These were appearing quite randomly though.  Upon further inspection
(and from my very limited understanding of NFS), I found that wcc data
is considered invalid if the real file's times are less or _equal_ to
the cached file's times.  This causes problems when a file is
accessed/created/whatever and the cached data is inserted on the exact
same nanosecond in the wcc.

How to reproduce that?  You only need to keep the nfs server and
client on the same machine.  tmpfs needn't be involved (although it
raises the errors much faster because it's internally simpler than ffs
and therefore has higher chances of finalizing in the same
nanosecond), as I've seen this pop up with ffs too.  Plus you do not
need a very fast machine; I see it within qemu on an Ahtlon XP 2600+.

The attached fix relaxes the condition checks to verifying if the
cached data is strictly older than the real one.  Is it correct?  Are
there any invalid situations in which a real file's time is equal to
the cached file's times and therefore wcc shall be disabled?

Thank you.

PS: BTW, what does wcc stand for?

-- 
Julio M. Merino Vidal <jmmv84@gmail.com>
The Julipedia - http://julipedia.blogspot.com/

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