Re: [PATCH] mac80211: Fix deadlock in ieee80211_do_stop.

[Johannes Berg <johannes@sipsolutions.net>]

On Fri, 2010-11-12 at 12:57 -0800, Ben Greear wrote:

> > However, I also don't think it should be necessary to do this.
> > sdata->work is always queued on local->workqueue, which is created using
> > alloc_ordered_workqueue(), and there is no work on this workqueue that
> > uses the RTNL. Therefore, even flushing the entire workqueue must work,
> > unless alloc_ordered_workqueue() has no such guarantee any more -- which
> > I would consider to be a bug in the new workqueue framework.
> 
> The problem appears (to me) to be that the flush_work() attempts
> to wait for the worker to complete it's current task.  The worker can
> be doing a completely separate task (ie, wireless_nlevent_process),
> but that task can never complete because do_stop() holds rtnl
> and the task-in-progress may block on acquiring rtnl.
> 
> So, flush_work() cannot make any progress.
> 
> The stack-traces for hung programs I originally posted seem
> to agree with this analysis.
> 
> So far, I reproduced the bug around 20 times in a row witout the patch,
> and since I added this patch, I have two good runs in a row, so it definitely
> has an affect.
> 
> If my assumptions are correct, it would seem to unsafe to EVER
> call flush_work() while holding rtnl (or indeed, any other lock
> that any other work could possibly require).

Well then in that case all I'm saying is that we have a bug in the
workqueue code, because it used to be allowed to flush a workqueue that
never had any work items on it that grabbed the RTNL themselves. I
actually added lockdep detection for the case where you _did_, but since
I'm sure we don't have anything that grabs the RTNL on our workqueue,
this should be OK.

johannes