Re: Pinning down a blocked task to extract diagnostics

[Paul E. McKenney]

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) {
				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);
	} else {
		/* Woo-hoo!  It started running again!!! */
	}

Is there a better way to approach this?

							Thanx, Paul