Re: Pinning down a blocked task to extract diagnostics

[Peter Zijlstra <peterz@infradead.org>]

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.