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Date:	Mon, 28 Jul 2014 14:54:43 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <cl@...two.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tejun Heo <tj@...nel.org>, John Stultz <johnstul@...ibm.com>,
	Mike Frysinger <vapier@...too.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, hughd@...gle.com,
	viresh.kumar@...aro.org, hpa@...or.com, mingo@...nel.org,
	peterz@...radead.org
Subject: Re: vmstat: On demand vmstat workers V8

On Mon, 28 Jul 2014 13:55:17 -0500 (CDT) Christoph Lameter <cl@...two.org> wrote:

> On Fri, 25 Jul 2014, Sasha Levin wrote:
> 
> > This patch doesn't interact well with my fuzzing setup. I'm seeing
> > the following:
> >
> > [  490.446927] BUG: using __this_cpu_read() in preemptible [00000000] code: kworker/16:1/7368
> > [  490.447909] caller is __this_cpu_preempt_check+0x13/0x20
> 
> __this_cpu_read() from vmstat_update is only called from a kworker that
> is bound to a single cpu. A false positive?

schedule_delayed_work() uses system_wq.  The comment in workqueue.h says

 * system_wq is the one used by schedule[_delayed]_work[_on]().
 * Multi-CPU multi-threaded.  There are users which expect relatively
 * short queue flush time.  Don't queue works which can run for too
 * long.

but the code itself does

	system_wq = alloc_workqueue("events", 0, 0);

ie: it didn't pass WQ_UNBOUND in the flags.


Tejun, wazzup?



Also, Sasha's report showed this:

[  490.464613] kernel BUG at mm/vmstat.c:1278!

That's your VM_BUG_ON() in vmstat_update().  That ain't no false
positive!



Is this code expecting that schedule_delayed_work() will schedule the
work on the current CPU?  I don't think it will do that.  Maybe you
should be looking at schedule_delayed_work_on().

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