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Date: Thu, 4 Feb 2016 07:37:24 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Tejun Heo <tj@kernel.org>
Cc: Jiri Slaby <jslaby@suse.cz>, Thomas Gleixner <tglx@linutronix.de>,
        Petr Mladek <pmladek@suse.com>, Jan Kara <jack@suse.cz>,
        Ben Hutchings <ben@decadent.org.uk>,
        Sasha Levin <sasha.levin@oracle.com>, Shaohua Li <shli@fb.com>,
        LKML <linux-kernel@vger.kernel.org>, stable@vger.kernel.org,
        Daniel Bilik <daniel.bilik@neosystem.cz>
Subject: Re: Crashes with 874bbfe600a6 in 3.18.25
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On Wed 03-02-16 11:59:01, Tejun Heo wrote:
> On Wed, Feb 03, 2016 at 05:48:52PM +0100, Michal Hocko wrote:
[...]
> > anything and add_timer_on also for WORK_CPU_UNBOUND is really required
> > then we should at least preserve WORK_CPU_UNBOUND in dwork->cpu so that
> > __queue_work can actually move on to the local CPU properly and handle
> > the offline cpu properly.
> 
> delayed_work->cpu is determined on queueing time.  Dealing with
> offlined cpus at execution is completley fine.  There's no need to
> "preserve" anything.

I've seen you have posted a fix in the mean time but just for my
understading. Why the following is not an appropriate fix?

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c579dbab2e36..52bb11cf20d1 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1459,9 +1459,9 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
 
 	dwork->wq = wq;
 	/* timer isn't guaranteed to run in this cpu, record earlier */
+	dwork->cpu = cpu;
 	if (cpu == WORK_CPU_UNBOUND)
 		cpu = raw_smp_processor_id();
-	dwork->cpu = cpu;
 	timer->expires = jiffies + delay;
 
 	add_timer_on(timer, cpu);

Thanks!
-- 
Michal Hocko
SUSE Labs