Subject: Re: [PATCH 1/2] workqueue: Catch more locking problems with flush_work()

From: Yong Zhang <[email-masked]>
Date: Thu, 19 Apr 2012 16:10:02 +0800

On Wed, Apr 18, 2012 at 08:25:57PM -0700, Stephen Boyd wrote:
> If a workqueue is flushed but the work item is not scheduled to
> run, lockdep checking will be circumvented. For example:
> 
>  static DEFINE_MUTEX(mutex);
> 
>  static void my_work(struct work_struct *w)
>  {
>          mutex_lock(&mutex);
>          mutex_unlock(&mutex);
>  }
> 
>  static DECLARE_WORK(work, my_work);
> 
>  static int __init start_test_module(void)
>  {
>          schedule_work(&work);
>          return 0;
>  }
>  module_init(start_test_module);
> 
>  static void __exit stop_test_module(void)
>  {
>          mutex_lock(&mutex);
>          flush_work(&work);
>          mutex_unlock(&mutex);
>  }
>  module_exit(stop_test_module);
> 
> would only print a warning if the work item was actively running
> when flush_work() was called. Otherwise flush_work() returns
> early. In this trivial example nothing could go wrong, but if the
> work item is schedule via an interrupt we could potentially have a
> scenario where the work item is running just at the time flush_work()

You mean flush_work() could be called in interupt? I don't it is
possible.

> is called. This could become a classic AB-BA locking problem.

I don't see how the deadlock happen, could you please be more
specific?

Thanks,
Yong

> 
> Add a lockdep hint in flush_work() in the "work not running" case
> so that we always catch this potential deadlock scenario.
> 
> Signed-off-by: Stephen Boyd <[email-masked]>
> ---
>  kernel/workqueue.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 66ec08d..eb800df 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2513,8 +2513,11 @@ bool flush_work(struct work_struct *work)
>  		wait_for_completion(&barr.done);
>  		destroy_work_on_stack(&barr.work);
>  		return true;
> -	} else
> +	} else {
> +		lock_map_acquire(&work->lockdep_map);
> +		lock_map_release(&work->lockdep_map);
>  		return false;
> +	}
>  }
>  EXPORT_SYMBOL_GPL(flush_work);
>  
> -- 
> Sent by an employee of the Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
> 
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