Re: [PATCH 6/6] freezer: kill unused set_freezable_with_signal()

[Matt Helsley <matthltc@us.ibm.com>]

On Tue, Sep 06, 2011 at 12:28:55PM +0900, Tejun Heo wrote:
> Hello,
> 
> On Mon, Sep 05, 2011 at 06:20:12PM +0200, Oleg Nesterov wrote:
> > Perhaps it is correct... Just I do not understand what it should do.
> > I thought it is "wait_for_event && do_not_block_freezer". And at first
> > glance the code looks as if it tries to do this. Say, in the "likely"
> > case we restart wait_event_interruptible() after refrigerator().
> > 
> > But this looks racy. Suppose that freezing() is already false when
> > try_to_freeze() or __refrigerator() is called. Say, cgroup_freezer does
> > freeze_task() + __thaw_task(). Why it returns -ERESTARTSYS in this case?
> 
> It may return -ERESTARTSYS when not strictly necessary but given that
> it's supposed to trigger restart anyway I don't think it's actually
> broken (it doesn't break the contract of the wait).  Another thing to
> note is that kthread freezing via cgroup is almost fundamentally
> broken.  With the removal of freezable_with_signal, this shouldn't be
> an issue anymore although cgroup_freezer still needs to be fixed to
> account for kthreads for xstate transitions (it currently triggers
> BUG_ON).

Looking at the code and docs I realize I didn't explicitly mention that
kthreads could not be frozen by the cgroup freezer. However, the code did
not allow it. When freezing tasks the cgroup freezer always did:

	freeze_task(task, true)

which would only freeze tasks without PF_FREEZER_NOSIG due to the second
"sig_only" parameter. I believe this means it could not be used to
freeze kthreads.

My feeling is kthreads should not be "managed" directly by userspace.
Especially if that includes the ability to arbitrarily stop or freeze them.
So it seems more appropriate to explicitly disallow freezing of kthreads
via the cgroup freezer.

Cheers,
	-Matt Helsley