Re: [PATCH 12/13] loop: remove lo_refcount and avoid lo_mutex in ->open / ->release

[Jan Kara <jack@suse.cz>]

On Tue 29-03-22 15:14:27, Christoph Hellwig wrote:
> On Tue, Mar 29, 2022 at 11:42:03AM +0200, Jan Kara wrote:
> > > entirely, as loop_clr_fd now is safe against concurrent users - it
> > > has to anyway as there can be other users even without an open.
> > 
> > Ah, OK, so you'd always set LO_AUTOCLEAR and leave cleanup to happen
> > from lo_release()? That makes sense to me.
> 
> No, my idea was to never set LO_AUTOCLEAR.  We have a frozen queue and
> all protections in place to make clearing the file perfectly safe.
> In fact the change_fd case also allows this.

I see, thanks for explanation. But then there's the risk of userspace
regressions if someone relies on the current behavior that LOOP_CLR_FD
ioctl does only delayed teardown of the device if someone is using it.
Personally I don't think the risk of regression is worth the benefit of the
cleanup but maybe it's worth trying. At least I know that for loop device
handling inside mount(8) (which plays tricks with reusing existing bound
loop devices), this change should be safe.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR