Re: [PATCH 01/10] VFS: support parallel updates in the one directory.

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, Sep 03, 2022 at 03:12:26AM +0100, Al Viro wrote:

> Very much so.  You are starting to invent new rules for ->lookup() that
> just never had been there, basing on nothing better than a couple of
> examples.  They are nowhere near everything there is.

A few examples besides NFS and autofs:

ext4, f2fs and xfs might bloody well return NULL without hashing - happens
on negative lookups with 'casefolding' crap.

kernfs - treatment of inactive nodes.

afs_dynroot_lookup() treatment of @cell... names.

afs_lookup() treatment of @sys... names.

There might very well be more - both merged into mainline and in
development trees of various filesystems (including devel branches
of in-tree ones - I'm not talking about out-of-tree projects).

Note, BTW, that with the current rules it's perfectly possible to
have this kind of fun:
	a name that resolves to different files for different processes
	unlink(2) is allowed and results depend upon the calling process

All it takes is ->lookup() deliberately *NOT* hashing the sucker and
->unlink() acting according to dentry it has gotten from the caller.
unlink(2) from different callers are serialized and none of that
stuff is ever going to be hashed.  d_alloc_parallel() might pick an
in-lookup dentry from another caller of e.g. stat(2), but it will
wait for in-lookup state ending, notice that the sucker is not hashed,
drop it and retry.  Suboptimal, but it works.

Nothing in the mainline currently does that.  Nothing that I know of,
that is.  Sure, it could be made work with the changes you seem to
imply (if I'm not misreading you) - all it takes is lookup
calling d_lookup_done() on its argument before returning NULL.
But that's subtle, non-obvious and not documented anywhere...

Another interesting question is the rules for unhashing dentries.
What is needed for somebody to do temporary unhash, followed by
rehashing?