Re: [PATCH w-t] iwlwifi: rewrite iwl-scan.c to avoid race conditions

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Wed, Sep 01, 2010 at 02:41:52PM +0200, Johannes Berg wrote:
> > Custom workqueue is not strictly needed, but it assure all works will
> > run short after schedule. Common workqueue can not give us such guarantees,
> > as other driver/subsystem can schedule own work, possibly slow, which
> > can block start of our work for long time.
> 
> I believe that's no longer true, with Tejun's workqueue rewrite that
> just got into mainline.

Good. However I still think having separate workqueue for scanning is clean
and consistent solution, I prefer it over schedule_work() ... and want
that patch backport to RHEL6 2.6.32 where we do not have such goodies :-)
 
> > > Shouldn't that only be a few of them anyway?
> > 
> > For sure abort_scan and abort_timeout works have to be scheduled on something
> > other than priv->workqueue. I'm queuing all scan works on priv->scan_workqueue
> > for consistency.
> 
> Remind me: The reason is that we need to cancel them from within the
> workqueue? But if we're on the same workqueue, it seems like they
> couldn't be running already, so cancel_work_sync() would always cancel
> them? 

Problem is not canceling, but exactly that we can not run new work when
old one does not finish. For example, if queued to priv->workqueue
abort_timeout will not be able run when we are performing iwl_bg_restart.
Will run after iwl_bg_restart finish, we don't want that.

> Do we get lockdep errors for that? Or are there actual locks
> involved?

No, priv->mutex used to protect critical sections of functions,
but it's not keep all the time in works (i.e: iwl_bg_restart),
so give scan code chance to complete.

Stanislaw