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SubjectRe: Pinning down a blocked task to extract diagnostics
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.

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