On 11/12/2010 01:08 PM, Johannes Berg wrote:
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.
You may well be right. The current behaviour makes
flush_work() pretty tricky to use correctly.
I'll let the folks that know work-queues better be
the judge.
Thanks,
Ben
johannes
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Ben Greear <greearb@xxxxxxxxxxxxxxx>
Candela Technologies Inc http://www.candelatech.com
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