Re: [PATCH wq/for-3.6-fixes 3/3] workqueue: fix possible idle worker depletion during CPU_ONLINE

[Tejun Heo <tj@kernel.org>]

Hello again, Lai.

On Fri, Sep 07, 2012 at 12:29:39PM -0700, Tejun Heo wrote:
> > Since we introduce manage_mutex(), any palace should be allowed to grab it
> > when its context allows. So it is not hotplug code's responsibility of this bug.
> > 
> > manage_workers() just use mutex_trylock() to grab the lock, it does not make
> > hard to do it jobs when need, and it does not try to find out the reason of fail.
> > so I think it is manage_workers()'s responsibility to handle this bug.
> > a manage_workers_slowpath() is enough to fix the bug.
> 
> It doesn't really matter how the synchronization between regular
> manager and hotplug path is done.  The point is that hotplug path, as
> much as possible, should be responsible for any incurred complexities,
> so I'd really like to stay away from adding a completely different
> path manager can be invoked in the usual path if at all possible.
> Let's try to solve this from the hotplug side.

So, how about something like the following?

* Make manage_workers() called outside gcwq->lock (or drop gcwq->lock
  after checking MANAGING).  worker_thread() can jump back to woke_up:
  instead.

* Distinguish synchronization among workers and against hotplug.  Was
  this what you tried with non_manager_mutex?  Anyways, revive
  WORKER_MANAGING to synchronize among workers.  If the worker won
  MANAGING, drop gcwq->lock and mutex_lock() gcwq->hotplug_mutex and
  then do other stuff.

This should prevent any idle worker passing through manage_workers()
while hotplug is in progress.  Do you think it would work?

Thanks.

-- 
tejun