Re: [RFC] MNT_WRITE_HOLD mess

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Jul 04, 2025 at 09:23:37PM +0100, Al Viro wrote:
> On Fri, Jul 04, 2025 at 12:57:39PM -0700, Linus Torvalds wrote:
> 
> > Ugh. I don't hate the concept, but if we do this, I think it needs to
> > be better abstracted out.
> > 
> > And you may be right that things like list_for_each_entry() won't
> > care, but I would not be surprised there is list debugging code that
> > could care deeply. Or if anybody uses things like "list_is_first()",
> > it will work 99+_% of the time, but then break horribly if the low bit
> > of the prev pointer is set.
> > 
> > So we obviously use the low bits of pointers in many other situations,
> > but I do think that it needs to have some kind of clear abstraction
> > and type safety to make sure that people don't use the "normal" list
> > handling helpers silently by mistake when they won't actually work.
> 
> Point, but in this case I'd be tempted to turn the damn thing into
> pointer + unsigned long right in the struct mount, and deal with it
> explicitly.  And put a big note on it, along the lines of "we might want
> to abstract that someday".
> 
> Backporting would be easier that way, if nothing else...

FWIW, several observations around that thing:
	* mnt_get_write_access(), vfs_create_mount() and clone_mnt()
definitely do not need to touch the seqcount component of mount_lock.
read_seqlock_excl() is enough there.
	* AFAICS, the same goes for sb_prepare_remount_readonly(),
do_remount() and do_reconfigure_mnt() - no point bumping the seqcount
side of mount_lock there; only spinlock is needed.
	* failure exit in mount_setattr_prepare() needs only clearing the
bit; smp_wmb() is pointless there (especially done for each mount involved).
	* both vfs_create_mount() and clone_mnt() add mount to the tail
of ->s_mounts.  Is there any reason why that would be better than adding
to head?  I don't remember if that had been covered back in 2010/2011
discussion of per-mount r/o patchset; quick search on lore hasn't turned
up anything...  Miklos?