linux-kernel - Re: [PATCH v3] virtio_balloon: Convert "vballoon" kthread into a workqueue

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Date:	Thu, 20 Nov 2014 11:49:33 -0500
From:	Tejun Heo <tj@...nel.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Petr Mladek <pmladek@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jeff Epler <jepler@...ythonic.net>,
	Jiri Kosina <jkosina@...e.cz>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] virtio_balloon: Convert "vballoon" kthread into a
 workqueue

On Thu, Nov 20, 2014 at 06:47:11PM +0200, Michael S. Tsirkin wrote:
> > There's cancel_work_sync() to stop the self-requeueing ones.
> 
> What happens if queue_work runs while cancel_work_sync is in progress?
> Does it fail to queue?

cancel_work_sync() is guaranteed to take self-requeueing work items no
matter when it's called or what's going on.  External (non-self)
queueings of course should be stopped in other ways.

> > > From that POV a dedicated WQ kept it simple.
> > 
> > A dedicated wq doesn't do anything for that.  You can't shut down a
> > workqueue with a pending work item on it.  destroy_workqueue() will
> > try to drain the target wq, warn if it doesn't finish in certain
> > number of iterations and just keep trying indefinitely.
> > 
> > Thanks.
> 
> Right, so eventually we'll stop requeueuing and it will succeed?

Yeah, sure, it's a silly reason to use a separate workqueue tho.
Don't do it that way.

Thanks.

-- 
tejun
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