linux-ext4 - 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

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Date: Mon, 27 Nov 2023 18:26:11 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Gabriel Krisman Bertazi <gabriel@...sman.be>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>, tytso@....edu,
	linux-f2fs-devel@...ts.sourceforge.net, ebiggers@...nel.org,
	linux-fsdevel@...r.kernel.org, jaegeuk@...nel.org,
	linux-ext4@...r.kernel.org, Miklos Szeredi <miklos@...redi.hu>
Subject: 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

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@...ssion.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).