Well, d_lookup() isn't the only place that does a dget() without
holding dcache_lock. There are *many* places where dget() is
done without holding dcache_lock. That didn't seem to be a
requirement in the pre-RCU dcache model.
> In most cases, the "do some operation" above is
I don't think that would work in pre or post-RCU dcache.
> in order (for instance) to ensure that nobody else can look up this
> dentry while we're working on it (e.g. rename or unlink,...).
rename, unlink etc. hold the per-dentry lock, so they are protected
against lockfree d_lookup().
> Your d_lookup() screws the above example of code which you can find in
> any number of VFS functions. dput(), d_delete(), d_invalidate(),
> d_prune_aliases(), prune_dcache(), shrink_dcache_sb() are but a few
> functions that rely on the above code snippet working to keep
> d_lookup() from intruding.
Those routines hold the per-dentry lock as required and that protects
them from intruding lockfree d_lookup().
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