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SubjectRe: Pinning down a blocked task to extract diagnostics
On Thu, Mar 05, 2020 at 09:28:45AM -0500, Steven Rostedt wrote:
> On Thu, 5 Mar 2020 06:22:45 -0800
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
>
> > On Thu, Mar 05, 2020 at 09:13:37AM +0100, Peter Zijlstra wrote:
> > > 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.
> >
> > How about if I add something like this, located right by try_to_wake_up()?
> >
> > bool try_to_keep_sleeping(struct task_struct *t)
> > {
> > raw_spin_lock_irq(&t->pi_lock);
> > switch (t->state) {
> > case TASK_RUNNING:
> > case TASK_WAKING:
> > raw_spin_unlock_irq(&t->pi_lock);
> > return false;
> >
> > default:
> > if (t->on_rq) {
>
> Somehow I think there still needs to be a read barrier before the test to
> on_rq.

This is nowhere near a fastpath, so if there is uncertainty it gets
the smp_rmb(). Or an smp_load_acquire() on t->state.

> > raw_spin_unlock_irq(&t->pi_lock);
> > return false;
> > }
> >
> > /* OK to extract consistent diagnostic information. */
> > return true;
> > }
> > /* NOTREACHED */
> > }
> >
> > Then a use might look like this:
> >
> > if (try_to_keep_sleeping(t))
> > /* Extract consistent diagnostic information. */
> > raw_spin_unlock_irq(&t->pi_lock);
>
> Perhaps we should have a allow_awake(t) to match it?
>
> allow_awake(t);
>
> Where we have:
>
> static inline allow_awake(struct task_struct *t)
> {
> raw_spin_unlock_irq(&t->pi_lock);
> }

Makes sense to me!

Thanx, Paul

> -- Steve?
>
> > } else {
> > /* Woo-hoo! It started running again!!! */
> > }
> >
> > Is there a better way to approach this?
> >
> > Thanx, Paul
>

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