Re: Pinning down a blocked task to extract diagnostics

[Peter Zijlstra <peterz@infradead.org>]

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:

	raw_spin_lock_irq(&t->pi_lock);
	switch (t->state) {
	case TASK_RUNNING:
	case TASK_WAKING:
		/* ignore */
		break;

	default:
		/* 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.