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

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Message-ID: <20200304225226.6d631baa@oasis.local.home>
Date:   Wed, 4 Mar 2020 22:52:26 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     mingo@...hat.com, peterz@...radead.org, 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 Wed, 4 Mar 2020 16:50:49 -0800
"Paul E. McKenney" <paulmck@...nel.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