Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way

[Rafael J. Wysocki]

On Saturday, 12 May 2007 10:16, Gautham R Shenoy wrote:
> On Sat, May 12, 2007 at 02:01:41AM +0200, Rafael J. Wysocki wrote:
> > 
> > > So you migt as well not return any value at all, since the returned value 
> > > is apparently meaningless once the lock has been released.
> > 
> > No, it is not meaningless.
> 
> Right.
> 
> Agreed that the returned value might not necessarily reflect the
> current state of thread in question. Can it pose any problems?
> 
> Case : is_user_space() returns 0: 
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> We think the thread is a kernel thread.
> So we don't count this thread when the case is FREEZER_USER_SPACE. 
> 
> Now this thread can perform an execve() and enter the 
> userspace. It might not freeze.
> So we would be declaring that all the userspace threads have been frozen,
> when actually we might have this one li'l unfrozen devil from the 
> userspace. 
> 
> Do we care? 
> For cpu hotplug, I don't think so.
> 
> Because, like Oleg pointed out,  we know we'll anyway freeze all the 
> leftover threads while handling the case FREEZER_KERNEL_THREADS.
> 
> But I am not sure if this is the case with suspend/hibernate, since we
> need to do a sys_sync() between try_freeze_tasks(FREEZE_USER_SPACE) and
> try_to_freeze_tasks(FREEZE_KERNEL_THREADS).

>From the point of view of syncing it's only necessary to make sure that we
won't freeze a kernel thread that's needed for the syncing.  We can have an
additional user space task running at this point.

> Case:is_user_space() returns 1:
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> We think the thread is from userspace.
> We count this thread when the case is FREEZER_USER_SPACE. Now the
> thread enters the kernel space by either doing a daemonize() or exit().
> 
> The freezer code loops *atleast* twice before declaring the system
> frozen. So if this metamorphosis occurs between iteration i and
> iteration i+1, in i+1, we note that this thread is now a kernel thread,
> and don't count it anymore.
> 
> However, between the 'i'th and 'i+1'th interation, if this thread calls
> try_to_freeze(), it'll enter the refrigerator. We now have a cold
> kernel thread when we were trying to freeze only userspace.

Hmm, I'm not sure if the task can call try_to_freeze() after doing exit().
May it happen?

> So, the question should be, are we clearing the TIF_FREEZING flag in places
> where the thread can become a kernel thread and eventually call try_to_freeze.
> I won't expect an exiting thread to call try_to_freeze, but a
> daemonize()ed thread can.
> 
> So should we perform that check in reparent_to_kthreadd() ?
> We are protected by the tasklist_lock there, no?

Yes.  Still, I think the daemonize()ed threads should clear their TIF_FREEZE
flag unconditionally right after they have called exit_mm().  So that would be
in daemonize().

Or, perhaps, it's better to clear TIF_FREEZE (unconditionally) in exit_mm(),
after we've done tsk->mm = NULL?  Oleg, what do you think?

Greetings,
Rafael 
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