Re: [heads-up] buggered refcounting logics in cgroup1_mount()

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

On Fri, Jan 11, 2019 at 12:54:51PM -0800, Tejun Heo wrote:
> Hello, Al.
> 
> On Fri, Jan 11, 2019 at 07:23:09AM +0000, Al Viro wrote:
> > 	AFAICS, a cleaner solution would be this:
> > * to hell with kernfs_pin_sb(); just try to grab a reference to
> > cgroup_root on reuse.
> > * have cgroup_kill_sb() treat "it's already marked killed" as
> > "just drop the reference, then".
> > * after cgroup_do_mount() check if cgroup_root got marked killed and
> > do deactivate_locked_super() in such case (with the same
> > restart_syscall() failure exit).
> > 
> > 	Objections?  I would love to kill kernfs_pin_sb() as
> > a followup (it's a fundamentally broken API), but that's not
> > a stable fodder; some fix of refcounting bugs, OTOH, should be.
> 
> cgroup1 hierarchies have really weird set of requirements and the
> implementation has always been somewhat broken.  I thought I fixed it
> but obviously not.  I have no objection whatsoever and would much
> appreciate the work.

See vfs.git #fixes (the last two commits in there).  It seems to
work here, but I don't have the CRIU regression tests, etc., so
I would really appreciate if that thing got a real beating.