Re: Pinning down a blocked task to extract diagnostics

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Mar 04, 2020 at 10:52:26PM -0500, Steven Rostedt wrote:
> 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.

Thank you, Steve!

							Thanx, Paul