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SubjectRe: Pinning down a blocked task to extract diagnostics
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.

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