linux-kernel - Re: [PATCH v1 3/3] cgroup: relax common ancestor restriction for direct descendants

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Date:	Thu, 21 Jul 2016 08:34:36 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Aleksa Sarai <asarai@...e.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Aditya Kali <adityakali@...gle.com>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	Christian Brauner <cbrauner@...e.de>, dev@...ncontainers.org
Subject: Re: [PATCH v1 3/3] cgroup: relax common ancestor restriction for
 direct descendants

On Thu, 2016-07-21 at 11:26 -0400, Tejun Heo wrote:
> Hello, James.
> 
> On Thu, Jul 21, 2016 at 08:16:34AM -0700, James Bottomley wrote:
> > > That'd be one side.  The other side is the one moving.  Let's say 
> > > the system admin thing wants to move all processe from A proper 
> > > to B.   It would do that by draining processes from A's procs 
> > > file into B's and even that is multistep and can race.
> > 
> > So the second part is that once we allow the creation of
> > subdirectories, there's no unified tasks file, so there's no way of
> > draining A proper without enumerating and descending into the 
> > cgroupns created subtrees in A?
> 
> Not that.  If it races, it will end up moving processes which are no
> longer in A proper.

So if I as the cgroup ns owner am moving a task from A to A_subdir, the
admin scanning tasks in all of A may miss this task in motion because
all the tasks files can't be scanned atomically?

James