Re: Pinning down a blocked task to extract diagnostics

[Steven Rostedt]

On Thu, 5 Mar 2020 06:22:45 -0800
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:

> On Thu, Mar 05, 2020 at 09:13:37AM +0100, Peter Zijlstra wrote:
> > On Thu, Mar 05, 2020 at 09:07:55AM +0100, Peter Zijlstra wrote:  
> > > On Wed, Mar 04, 2020 at 04:50:49PM -0800, Paul E. McKenney wrote:  
> > > > Hello!
> > > > 
> > > > Suppose that I need to extract diagnostics information from a blocked
> > > > task, but that I absolutely cannot tolerate this task awakening in the
> > > > midst of this extraction process.  Is the following code the right way
> > > > to make this work given a task "t"?
> > > > 
> > > > 	raw_spin_lock_irq(&t->pi_lock);
> > > > 	if (t->on_rq) {
> > > > 		/* Task no longer blocked, so ignore it. */
> > > > 	} else {
> > > > 		/* Extract consistent diagnostic information. */
> > > > 	}
> > > > 	raw_spin_unlock_irq(&t->pi_lock);
> > > > 
> > > > It looks like all the wakeup paths acquire ->pi_lock, but I figured I
> > > > should actually ask...  
> > > 
> > > Close, the thing pi_lock actually guards is the t->state transition *to*
> > > TASK_WAKING/TASK_RUNNING, so something like this:  
> > 
> > Almost, we must indeed also check ->on_rq, otherwise it might change the
> > state back itself.
> >   
> > > 
> > > 	raw_spin_lock_irq(&t->pi_lock);
> > > 	switch (t->state) {
> > > 	case TASK_RUNNING:
> > > 	case TASK_WAKING:
> > > 		/* ignore */
> > > 		break;
> > > 
> > > 	default:  
> > 		if (t->on_rq)
> > 			break;
> >   
> > > 		/* Extract consistent diagnostic information. */
> > > 		break;
> > > 	}
> > > 	raw_spin_unlock_irq(&t->pi_lock);
> > > 
> > > ought to work. But if you're going to do this, please add a reference to
> > > that code in a comment on top of try_to_wake_up(), such that we can
> > > later find all the code that relies on this.  
> 
> How about if I add something like this, located right by try_to_wake_up()?
> 
> 	bool try_to_keep_sleeping(struct task_struct *t)
> 	{
> 		raw_spin_lock_irq(&t->pi_lock);
> 		switch (t->state) {
> 		case TASK_RUNNING:
> 		case TASK_WAKING:
> 			raw_spin_unlock_irq(&t->pi_lock);
> 			return false;
> 
> 		default:
> 			if (t->on_rq) {

Somehow I think there still needs to be a read barrier before the test to
on_rq.

> 				raw_spin_unlock_irq(&t->pi_lock);
> 				return false;
> 			}
> 
> 			/* OK to extract consistent diagnostic information. */
> 			return true;
> 		}
> 		/* NOTREACHED */
> 	}
> 
> Then a use might look like this:
> 
> 	if (try_to_keep_sleeping(t))
> 		/* Extract consistent diagnostic information. */
> 		raw_spin_unlock_irq(&t->pi_lock);

Perhaps we should have a allow_awake(t) to match it?

		allow_awake(t);

Where we have:

static inline allow_awake(struct task_struct *t)
{
	raw_spin_unlock_irq(&t->pi_lock);
}

-- Steve?

> 	} else {
> 		/* Woo-hoo!  It started running again!!! */
> 	}
> 
> Is there a better way to approach this?
> 
> 							Thanx, Paul