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

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Message-ID: <20200305005049.GA21120@paulmck-ThinkPad-P72>
Date:   Wed, 4 Mar 2020 16:50:49 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de
Cc:     linux-kernel@...r.kernel.org
Subject: Pinning down a blocked task to extract diagnostics

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...

							Thanx, Paul