Re: fun with d_invalidate() vs. d_splice_alias() was Re: [f2fs-dev] [PATCH v6 0/9] Support negative dentries on case-insensitive ext4 and f2fs

Al Viro
2023-11-27T17:26:05Z

On Mon, Nov 27, 2023 at 10:01:34AM -0600, Eric W. Biederman wrote:
> "Eric W. Biederman" <ebiederm@xmission.com> writes:
>
> > I am confused what is going on with ext4 and f2fs. I think they
> > are calling d_invalidate when all they need to call is d_drop.
>
> ext4 and f2f2 are buggy in how they call d_invalidate, if I am reading
> the code correctly.
>
> d_invalidate calls detach_mounts.
>
> detach_mounts relies on setting D_CANT_MOUNT on the top level dentry to
> prevent races with new mounts.
>
> ext4 and f2fs (in their case insensitive code) are calling d_invalidate
> before dont_mount has been called to set D_CANT_MOUNT.

Not really - note that the place where we check cant_mount() is under
the lock on the mountpoint's inode, so anything inside ->unlink() or
->rmdir() is indistinguishable from the places where we do dont_mount()
in vfs_{unlink,rmdir}.