linux-kernel - single-threaded wq lockdep is broken

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Date:   Sun, 28 May 2017 21:33:13 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Tejun Heo <tj@...nel.org>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: single-threaded wq lockdep is broken

Hi Tejun,

I suspect this is a long-standing bug introduced by all the pool rework
you did at some point, but I don't really know nor can I figure out how
to fix it right now. I guess it could possibly also be a lockdep issue,
or an issue in how it's used, but I definitely know that this used to
work (i.e. warn) back when I introduced the lockdep checking to the WQ
code. I was actually bitten by a bug like this, and erroneously
dismissed it as not being the case because lockdep hadn't warned (and
the actual deadlock debug output is basically not existent).

In any case, the following code really should result in a warning from
lockdep, but doesn't. If you comment in the #define DEADLOCK, it will
actually cause a deadlock :)

#include <linux/kernel.h>
#include <linux/mutex.h>
#include <linux/workqueue.h>
#include <linux/module.h>
#include <linux/delay.h>

DEFINE_MUTEX(mtx);
static struct workqueue_struct *wq;
static struct work_struct w1, w2;

static void w1_wk(struct work_struct *w)
{
	mutex_lock(&mtx);
	msleep(100);
	mutex_unlock(&mtx);
}

static void w2_wk(struct work_struct *w)
{
}

/*
 * if not defined, then lockdep should warn only,
 * if defined, the system will really deadlock.
 */

//#define DEADLOCK

static int init(void)
{
	wq = create_singlethread_workqueue("test");
	if (!wq)
		return -ENOMEM;
	INIT_WORK(&w1, w1_wk);
	INIT_WORK(&w2, w2_wk);

#ifdef DEADLOCK
	queue_work(wq, &w1);
	queue_work(wq, &w2);
#endif
	mutex_lock(&mtx);
	flush_work(&w2);
	mutex_unlock(&mtx);

#ifndef DEADLOCK
	queue_work(wq, &w1);
	queue_work(wq, &w2);
#endif

	return 0;
}
module_init(init);

(to test, just copy it to some C file and add "obj-y += myfile.o" to
the Makefile in that directory, then boot the kernel - perhaps in a VM)

johannes