linux-kernel - Re: Pinning down a blocked task to extract diagnostics

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Date:   Thu, 5 Mar 2020 09:01:36 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: Re: Pinning down a blocked task to extract diagnostics

On Thu, 5 Mar 2020 09:13:37 +0100
Peter Zijlstra <peterz@...radead.org> wrote:


> > 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:  

Don't we need a smp_rmb() here, otherwise we could have the same issue as
described in the comment in try_to_wake_up()?

-- Steve


> 		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.