[PATCH] fuse: fix conversion of fuse_reverse_inval_entry() to start_removing()

[Al Viro]

On Mon, Dec 01, 2025 at 03:03:08PM +0100, Miklos Szeredi wrote:
> On Mon, 1 Dec 2025 at 09:33, Al Viro <viro at zeniv.linux.org.uk> wrote:
> >
> > On Mon, Dec 01, 2025 at 09:22:54AM +0100, Amir Goldstein wrote:
> >
> > > I don't think there is a point in optimizing parallel dir operations
> > > with FUSE server cache invalidation, but maybe I am missing
> > > something.
> >
> > The interesting part is the expected semantics of operation;
> > d_invalidate() side definitely doesn't need any of that cruft,
> > but I would really like to understand what that function
> > is supposed to do.
> >
> > Miklos, could you post a brain dump on that?
> 
> This function is supposed to invalidate a dentry due to remote changes
> (FUSE_NOTIFY_INVAL_ENTRY).  Originally it was supplied a parent ID and
> a name and called d_invalidate() on the looked up dentry.
> 
> Then it grew a variant (FUSE_NOTIFY_DELETE) that was also supplied a
> child ID, which was matched against the looked up inode.  This was
> commit 451d0f599934 ("FUSE: Notifying the kernel of deletion."),
> Apparently this worked around the fact that at that time
> d_invalidate() returned -EBUSY if the target was still in use and
> didn't unhash the dentry in that case.
> 
> That was later changed by commit bafc9b754f75 ("vfs: More precise
> tests in d_invalidate") to unconditionally unhash the target, which
> effectively made FUSE_NOTIFY_INVAL_ENTRY and FUSE_NOTIFY_DELETE
> equivalent and the code in question unnecessary.
> 
> For the future, we could also introduce FUSE_NOTIFY_MOVE, that would
> differentiate between a delete and a move, while
> FUSE_NOTIFY_INVAL_ENTRY would continue to be the common (deleted or
> moved) notification.

Then as far as VFS is concerned, it's an equivalent of "we'd done
a dcache lookup and revalidate told us to bugger off", which does
*not* need locking the parent - the same sequence can very well
happen without touching any inode locks.

IOW, from the point of view of locking protocol changes that's not
a removal at all.

Or do you need them serialized for fuse-internal purposes?