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Date: Sat, 27 Jul 2013 22:41:44 +0530
From: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
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To: Tejun Heo <tj@kernel.org>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Rafael J. Wysocki" <rjw@sisk.pl>, bhelgaas@google.com,
        Yinghai Lu <yinghai@kernel.org>,
        Alex Duyck <alexander.h.duyck@intel.com>
Subject: Re: [PATCH] workqueue: allow work_on_cpu() to be called recursively
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On 07/24/2013 09:55 PM, Tejun Heo wrote:
> Applied to wq/for-3.11-fixes with comment and subject tweaks.
> 
> Thanks!
> 
> ---------- 8< ------------
> 
> From c2fda509667b0fda4372a237f5a59ea4570b1627 Mon Sep 17 00:00:00 2001
> From: Lai Jiangshan <laijs@cn.fujitsu.com>
> Date: Wed, 24 Jul 2013 18:31:42 +0800
> 
> If the @fn call work_on_cpu() again, the lockdep will complain:
> 
>> [ INFO: possible recursive locking detected ]
>> 3.11.0-rc1-lockdep-fix-a #6 Not tainted
>> ---------------------------------------------
>> kworker/0:1/142 is trying to acquire lock:
>>  ((&wfc.work)){+.+.+.}, at: [<ffffffff81077100>] flush_work+0x0/0xb0
>>
>> but task is already holding lock:
>>  ((&wfc.work)){+.+.+.}, at: [<ffffffff81075dd9>] process_one_work+0x169/0x610
>>
>> other info that might help us debug this:
>>  Possible unsafe locking scenario:
>>
>>        CPU0
>>        ----
>>   lock((&wfc.work));
>>   lock((&wfc.work));
>>
>>  *** DEADLOCK ***
> 
> It is false-positive lockdep report. In this sutiation,
> the two "wfc"s of the two work_on_cpu() are different,
> they are both on stack. flush_work() can't be deadlock.
> 
> To fix this, we need to avoid the lockdep checking in this case,
> thus we instroduce a internal __flush_work() which skip the lockdep.
> 
> tj: Minor comment adjustment.
> 
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> Reported-by: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
> Reported-by: Alexander Duyck <alexander.h.duyck@intel.com>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---

This version works as well, it fixes the issue I was facing.
Thank you!

FWIW:
Tested-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>

Regards,
Srivatsa S. Bhat

>  kernel/workqueue.c | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index f02c4a4..55f5f0a 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2817,6 +2817,19 @@ already_gone:
>  	return false;
>  }
> 
> +static bool __flush_work(struct work_struct *work)
> +{
> +	struct wq_barrier barr;
> +
> +	if (start_flush_work(work, &barr)) {
> +		wait_for_completion(&barr.done);
> +		destroy_work_on_stack(&barr.work);
> +		return true;
> +	} else {
> +		return false;
> +	}
> +}
> +
>  /**
>   * flush_work - wait for a work to finish executing the last queueing instance
>   * @work: the work to flush
> @@ -2830,18 +2843,10 @@ already_gone:
>   */
>  bool flush_work(struct work_struct *work)
>  {
> -	struct wq_barrier barr;
> -
>  	lock_map_acquire(&work->lockdep_map);
>  	lock_map_release(&work->lockdep_map);
> 
> -	if (start_flush_work(work, &barr)) {
> -		wait_for_completion(&barr.done);
> -		destroy_work_on_stack(&barr.work);
> -		return true;
> -	} else {
> -		return false;
> -	}
> +	return __flush_work(work);
>  }
>  EXPORT_SYMBOL_GPL(flush_work);
> 
> @@ -4756,7 +4761,14 @@ long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
> 
>  	INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
>  	schedule_work_on(cpu, &wfc.work);
> -	flush_work(&wfc.work);
> +
> +	/*
> +	 * The work item is on-stack and can't lead to deadlock through
> +	 * flushing.  Use __flush_work() to avoid spurious lockdep warnings
> +	 * when work_on_cpu()s are nested.
> +	 */
> +	__flush_work(&wfc.work);
> +
>  	return wfc.ret;
>  }
>  EXPORT_SYMBOL_GPL(work_on_cpu);
> 

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