Date: Mon, 28 Jul 2014 14:54:43 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Christoph Lameter <cl@gentwo.org>
Cc: Sasha Levin <sasha.levin@oracle.com>,
        Gilad Ben-Yossef <gilad@benyossef.com>,
        Thomas Gleixner <tglx@linutronix.de>, Tejun Heo <tj@kernel.org>,
        John Stultz <johnstul@us.ibm.com>, Mike Frysinger <vapier@gentoo.org>,
        Minchan Kim <minchan.kim@gmail.com>,
        Hakan Akkan <hakanakkan@gmail.com>,
        Max Krasnyansky <maxk@qualcomm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, hughd@google.com,
        viresh.kumar@linaro.org, hpa@zytor.com, mingo@kernel.org,
        peterz@infradead.org
Subject: Re: vmstat: On demand vmstat workers V8
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On Mon, 28 Jul 2014 13:55:17 -0500 (CDT) Christoph Lameter <cl@gentwo.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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