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 <viro@zeniv.linux.org.uk>]

On Mon, Nov 27, 2023 at 05:25:44PM +0000, Al Viro wrote:
> 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}.

Said that, we could simply use d_drop() in those, since the caller will
take care of mount eviction - we have ->unlink() or ->rmdir() returning
success, after all.

The same goes for xfs caller and for cifs_prime_dcache() (in the latter
case we have just checked that they sucker is negative, so d_invalidate()
and d_drop() are doing the same thing).