On Sun, 04 Sep 2022, Al Viro wrote: > 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: Hi Al, thanks for these - very helpful. I will give them due consideration when I write relevant documentation to include in the next posting of the series. Thanks a lot, NeilBrown > > 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? >
