Subject: Re: [PATCHv2] workqueue: Catch more locking problems with flush_work()

From: Tejun Heo <[email-masked]>
Date: Mon, 23 Apr 2012 11:07:57 -0700

On Fri, Apr 20, 2012 at 05:28:50PM -0700, Stephen Boyd wrote:
> If a workqueue is flushed with flush_work() lockdep checking can
> 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 not always print a warning when flush_work() was called.
> In this trivial example nothing could go wrong since we are
> guaranteed module_init() and module_exit() don't run concurrently,
> but if the work item is schedule asynchronously we could have a
> scenario where the work item is running just at the time flush_work()
> is called resulting in a classic ABBA locking problem.
> 
> Add a lockdep hint by acquiring and releasing the work item
> lockdep_map in flush_work() so that we always catch this
> potential deadlock scenario.
> 
> Signed-off-by: Stephen Boyd <[email-masked]>

Applied to wq/for-3.5.  Let's see whether it triggers spuriously.

Thanks.

-- 
tejun