Re: [PATCH 02/24] workqueue: defer work to a draining queue

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Hello,

On Fri, Dec 16, 2011 at 06:33:18PM -0800, Dan Williams wrote:
> commit 9c5a2ba7 "workqueue: separate out drain_workqueue() from
> destroy_workqueue()" provided drain_workqueue() for users like libsas to
> use for flushing events.
> 
> When libsas drains it wants currently queued and chained events to be
> flushed, but it fully expects to continue issuing unchained events with
> the expectation that they are serviced sometime after the drain.
> 
> For external users of drain_workqueue() arrange for unchained work to be
> queued after the drain completes, if the caller cares if unchained work
> was queued as a result of the drain it can check for a non-zero return
> value.  Deferred work is guaranteed to be at least queued when
> drain_workqueue() returns, and visible to flush_workqueue() users as
> well.
> 
> Unfortunately this causes the promotion of workqueue_lock to hard-irq
> safe and does not guarantee that work submitted via queue_work_on() runs
> on the specified cpu if it gets deferred.
> 
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Dan, as I replied before, I'm not a big fan of this approach.  For
now, I think it would be best to add private wrapper in libsas to
support deferring unchained work items while draining.

Thanks.

-- 
tejun
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