Re: Policy wrt mail lockfile (section 4.3)
Christian Schwarz wrote:
>AFAIK, there is at least one safe way to lock a file over NFS. The
>procedure is partially explained in the open(2) man page and is also
>implemented, for example, in your "publib" library.
I've dug deeper into the NFS protocol (RFC 1057 and RFC 1094) than is
good for me :-) and I don't think there is such a thing for NFS over UDP
without a lockd, which Linux doesn't have.
The problem is that any NFS request (CREATE, LINK, ...) can get lost, and
its ack can get lost, too. Let's take a CREATE request. If no ack comes
back, there are several cases the client can't tell apart:
a) Server never got the request.
b) Server got the request, but the lockfile already existed
(status = NFSERR_EXIST), and the reply got lost
c) Server got the request, and created the lockfile (status = NFS_OK),
but the reply got lost.
If the client retries and gets status=NFSERR_EXIST after not hearing
anything from the server, what should it assume that b) or c) happened
Yes, the server can do xid caching to a certain extent, but the
client can't rely on that.
If the protocol in the publib library has a way to get around that
problem, I'd be interesting in learning more about it (and, possibly,
dreaming up cases in which it might fail :-)
Thomas Koenig, Thomas.Koenig@ciw.uni-karlsruhe.de, firstname.lastname@example.org.
The joy of engineering is to find a straight line on a double
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