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SubjectRe: NFS locking bug -- limited mtime resolution means nfs_lock() does not provide coherency guarantee
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>      > The fix we have found is to *either* have NFS_CACHEINV(inode)
> > change inode-> i_mtime to an artificial value (0) or to call
> > nfs_zap_caches instead. Since I am not sure which fix is
> > appropriate, I'm not enclosing an actual patch.
>
> This is a known issue, and is not easy to fix. Neither of the
> solutions you propose are correct since they will both cause a cache
> invalidation. This is not the same as cache coherency checking.

Providing everyone is careful to hold a lock I think it is

lockf() is a read barrier providing the local cache is flushed, the unlock
is a write barrier providing the local cache is flushed first. Providing
all users are using lockf for their I/O then it seems to be coherent

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