Re: [PATCH v3 07/19] VFS: Add LOOKUP_SHARED flag.

[Amir Goldstein]

On Mon, Apr 27, 2026 at 6:06 AM NeilBrown <neilb@xxxxxxxxxxx> wrote:
>
> From: NeilBrown <neil@xxxxxxxxxx>
>
> Some ->lookup handlers will need to drop and retake the parent lock, so
> they can safely use d_alloc_parallel().
>
> ->lookup can be called with the parent lock either exclusive or shared.
>
> A new flag, LOOKUP_SHARED, tells ->lookup how the parent is locked.
>
> This is rather ugly, but will be gone soon after we move
> d_alloc_parallel() out of the directory lock as ->lookup() will *always*
> called with a shared lock on the parent.

Neil,

Forgive me for being skeptical about the *always* part.

How long ago did we add ->iterate_shared()?

It's true that Linus eventually got rid of ->iterate(), but we did not
get rid of the assumption that iterate_shared() might be upgraded
to exclusive lock.

The obvious reason is that *someone* needs to do this work for
old filesystems, which are also hard to test and nobody wants to
touch them.

I have nothing against this patch, but I think it is more realistic
to state that LOOKUP_SHARED is here to stay, so if you think it
is too ugly, maybe there is something to be done about it.
Personally, I do not see the ugliness though.

Am I misjudging the situation of shared lookup wrt old filesystems?

Thanks,
Amir.