On Tue, Dec 19, 2006 at 03:43:19AM +0300, Oleg Nesterov wrote:

quoted text
> > Taking workqueue_mutex() unconditionally in flush_workqueue() means

> > that we'll deadlock if a single-threaded workqueue callback handler calls

> > flush_workqueue().

>

> Well. But flush_workqueue() drops workqueue_mutex before going to sleep ?

... and acquires it again after woken from sleep. That can be a problem, which

will lead to the problem described here:
	http://lkml.org/lkml/2006/12/7/374
In brief:
keventd thread					hotplug thread

--------------					--------------
  run_workqueue()

	|

     work_fn()

	 |

	flush_workqueue()

	     |

	   flush_cpu_workqueue

		|				cpu_down()

	     mutex_unlock(wq_mutex);		     |

	(above opens window for hotplug)	   mutex_lock(wq_mutex);

    		|				   /* bring down cpu */

	     wait_for_completition();		     notifier(CPU_DEAD, ..)

		| 				       workqueue_cpu_callback

		| 				        cleanup_workqueue_thread

		|					  kthread_stop()

		|

		|

	     mutex_lock(wq_mutex); <- Can deadlock
The kthread_stop() will wait for keventd() thread to exit, but keventd()

is blocked on mutex_lock(wq_mutex) leading to a deadlock.
>

quoted text
> 	flush_workqueue(single_threaded_wq);

> 		...

> 		mutex_lock(&workqueue_mutex);

> 		...

> 		mutex_unlock(&workqueue_mutex);

> 		wait_for_completition();

> 							handler runs,

> 							calls flush_workqueue(),

> 							workqueue_mutex is free

--

Regards,

vatsa

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