Re: Freezeable workqueues [Was: 2.6.22-rc1: Broken suspend on SMP with tifm]

[Oleg Nesterov <oleg@tv-sign.ru>]

On 05/15, Rafael J. Wysocki wrote:
>
> On Monday, 14 May 2007 23:48, Oleg Nesterov wrote:
> > 
> > So, in the long term, should we change this only user, or we think we better fix
> > freezeable wqs again?
> 
> Long term, I'd like to have freezable workqueues, so that people don't have to
> use "raw" kernel threads only because they need some synchronization with
> hibertnation/suspend.  Plus some cases in which workqueues are used by
> fs-related code make me worry.

OK, so we should fix them. It would be great to also fix the last known problem
as well (work->func() vs hotplug callback deadlocks).

I am a bit afraid of too many yes/no options for the freezer, a couple of naive
questions.

1. Can't we make all wqs freezable? I still can't see the reason to have both
   freezable and not freezable wqs.

2. Why do we need CPU_TASKS_FROZEN? Can't we change cpu-hotplug to always
   freeze tasks right now, without any additional changes?

   Any subsystem should handle correctly the case when _cpu_down() (say)
   is called with tasks_frozen == 1 anyway. So, why can't we simplify
   things and do

   	_cpu_down(int tasks_frozen)

 		if (!tasks_frozen)
 			freeze_processes();
 		...

  right now?

> [*] The problem is, though, that freezable workqueus have some potential to fail
> the freezer.  Namely, suppose task A calls flush_workqueue() on a freezable
> workqueue, finds some work items in there, inserts the barrier and waits for
> completion (TASK_UNINTERRUPTIBLE).  In the meantime, TIF_FREEZE is set on
> the worker thread, which is then woken up and goes to the refrigerator.  Thus
> if A is not NOFREEZE, the freezing of tasks will fail (A must be a kernel
> thread for this to happen, but still).  Worse yet, if A is NOFREEZE, it will be
> blocked until the worker thread is woken up.

Yes, this is yet another dependency which freezer can't handle. Probably it is
better to ignore this problem for now.

> To avoid this, I think, we may need to redesign the freezer, so that freezable
> worker threads are frozen after all of the other kernel threads.

I doubt we can find a very clean way to do this. Besides, what if work->func()
does flush_workqueue(another_wq) ? How can we decide which wq to freeze first?

>                                                                   Additionally,
> we'd need to make a rule that NOFREEZE kernel threads must not call
> flush_workqueue() or cancel_work_sync() on freezable workqueues.

cancel_work_sync() is OK, it can be used safely even if workqueue is frozen.
flush_workqueue() and destroy_workqueue() are not.

Oleg.