Subject: Re: [RFC][PATCH] cgroup: Fix cgroup_subsys::exit callback
From: Peter Zijlstra <peterz@infradead.org>
To: Paul Menage <menage@google.com>
Cc: balbir@linux.vnet.ibm.com, eranian@google.com,
        linux-kernel@vger.kernel.org, mingo@elte.hu, paulus@samba.org,
        davem@davemloft.net, fweisbec@gmail.com, perfmon2-devel@lists.sf.net,
        eranian@gmail.com, robert.richter@amd.com, acme@redhat.com,
        lizf@cn.fujitsu.com
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Date: Mon, 07 Feb 2011 21:02:39 +0100
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On Mon, 2011-02-07 at 11:28 -0800, Paul Menage wrote:
> On Mon, Feb 7, 2011 at 8:10 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Make the ::exit method act like ::attach, it is after all very nearly
> > the same thing.
> 
> The major difference between attach and exit is that the former is
> only triggered in response to user cgroup-management action, whereas
> the latter is triggered whenever a task exits, even if cgroups aren't
> set up.

And the major likeness is that they both migrate a task from one cgroup
to another. You cannot simply ignore that.

> >  void cgroup_exit(struct task_struct *tsk, int run_callbacks)
> >  {
> > -       int i;
> >        struct css_set *cg;
> > +       int i;
> >
> > -       if (run_callbacks && need_forkexit_callback) {
> > -               /*
> > -                * modular subsystems can't use callbacks, so no need to lock
> > -                * the subsys array
> > -                */
> > -               for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
> > -                       struct cgroup_subsys *ss = subsys[i];
> > -                       if (ss->exit)
> > -                               ss->exit(ss, tsk);
> > -               }
> > -       }
> > +       mutex_lock(&cgroup_mutex);
> 
> NACK - cgroup_mutex is way too heavy to take in the task exit path.
> We'll need to find some other way to fix this if it's really needed.
> task->alloc_lock is also normally a valid thing to synchronize against
> cgroup moves, but I'd have to look at the exit path to see if it's
> still valid there.

If maybe you're like respond more often than about once every two months
I might actually care what you think. 

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