Re: [RFC][PATCH] freezer,sched: Rewrite core freezer logic

[Will Deacon]

On Thu, Jun 03, 2021 at 01:26:06PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 03, 2021 at 11:58:56AM +0100, Will Deacon wrote:
> > On Thu, Jun 03, 2021 at 12:35:22PM +0200, Peter Zijlstra wrote:
> > > On Wed, Jun 02, 2021 at 01:54:53PM +0100, Will Deacon wrote:
> 
> > > > > @@ -116,20 +173,8 @@ bool freeze_task(struct task_struct *p)
> > > > >  {
> > > > >  	unsigned long flags;
> > > > >  
> > > > >  	spin_lock_irqsave(&freezer_lock, flags);
> > > > > +	if (!freezing(p) || frozen(p) || __freeze_task(p)) {
> > > > >  		spin_unlock_irqrestore(&freezer_lock, flags);
> > > > >  		return false;
> > > > >  	}
> > > > 
> > > > I've been trying to figure out how this serialises with ttwu(), given that
> > > > frozen(p) will go and read p->state. I suppose it works out because only the
> > > > freezer can wake up tasks from the FROZEN state, but it feels a bit brittle.
> > > 
> > > p->pi_lock; both ttwu() and __freeze_task() (which is essentially a
> > > variant of set_special_state()) take ->pi_lock. I'll put in a comment.
> > 
> > The part I struggled with was freeze_task(), which doesn't take ->pi_lock
> > yet calls frozen(p).
> 
> Ah, I can't read... I assumed you were asking about __freeze_task().
> 
> So frozen(p) checks for p->state == TASK_FROZEN (and complicated), which
> is a stable state. Once you're frozen you stay frozen until thaw, which
> is after freezing per construction.
> 
> The tricky bit is __freeze_task(), that does take pi_lock. It checks for
> FREEZABLE and if set, changes to FROZEN. And this does in fact race with
> ttwu() and relies on pi_lock to serialize.
> 
> A concurrent wakeup (from a non-frozen task) can try and wake the task
> we're trying to freeze. If we win, ttwu will see FROZEN and ignore, if
> ttwu() wins, we don't see FREEZABLE and do another round of freezing.

Good, thanks. That's where I'd ended up. It means that nobody else better
try waking up FROZEN tasks!

> > > > > @@ -137,7 +182,7 @@ bool freeze_task(struct task_struct *p)
> > > > >  	if (!(p->flags & PF_KTHREAD))
> > > > >  		fake_signal_wake_up(p);
> > > > >  	else
> > > > > -		wake_up_state(p, TASK_INTERRUPTIBLE);
> > > > > +		wake_up_state(p, TASK_INTERRUPTIBLE); // TASK_NORMAL ?!?
> > > > >  
> > > > >  	spin_unlock_irqrestore(&freezer_lock, flags);
> > > > >  	return true;
> > > > > @@ -148,8 +193,8 @@ void __thaw_task(struct task_struct *p)
> > > > >  	unsigned long flags;
> > > > >  
> > > > >  	spin_lock_irqsave(&freezer_lock, flags);
> > > > > -	if (frozen(p))
> > > > > -		wake_up_process(p);
> > > > > +	WARN_ON_ONCE(freezing(p));
> > > > > +	wake_up_state(p, TASK_FROZEN | TASK_NORMAL);
> > > > 
> > > > Why do we need TASK_NORMAL here?
> > > 
> > > It's a left-over from hacking, but I left it in because anything
> > > TASK_NORMAL should be able to deal with spuriuos wakeups, something
> > > try_to_freeze() now also relies on.
> > 
> > I just worry that it might hide bugs if TASK_FROZEN is supposed to be
> > sufficient, as it would imply that we have some unfrozen tasks kicking
> > around. I dunno, maybe just a comment saying that everything _should_ be
> > FROZEN at this point?
> 
> I'll take it out. It really shouldn't matter.

Perfect.

Will