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SubjectRe: Pinning down a blocked task to extract diagnostics
On Wed, 4 Mar 2020 16:50:49 -0800
"Paul E. McKenney" <paulmck@kernel.org> 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...

IIUC, the rtmutex code uses pi_lock to tinker with the task while it is
blocked (not woken). But the on_rq test may not be correct. It appears
that can change without holding the task's pi_lock. I'm looking at the
ttwu_do_activate() code which modifies the t->on_rq without holding the
pi_lock. Seems you may need to check the p->state as well. See the
comment in try_to_wake_up() about testing on_rq vs the state.

-- Steve

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