linux-kernel - Re: [RFC v2 07/18] kthread: Allow to cancel kthread work

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Message-ID: <20151005110924.GL9603@pathway.suse.cz>
Date:	Mon, 5 Oct 2015 13:09:24 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	Vlastimil Babka <vbabka@...e.cz>,
	live-patching@...r.kernel.org, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 07/18] kthread: Allow to cancel kthread work

On Mon 2015-10-05 12:07:58, Petr Mladek wrote:
> On Fri 2015-10-02 15:24:53, Tejun Heo wrote:
> > Hello,
> > 
> > On Fri, Oct 02, 2015 at 05:43:36PM +0200, Petr Mladek wrote:
> > > IMHO, we need both locks. The worker manipulates more works and
> > > need its own lock. We need work-specific lock because the work
> > > might be assigned to different workers and we need to be sure
> > > that the operations are really serialized, e.g. queuing.
> > 
> > I don't think we need per-work lock.  Do we have such usage in kernel
> > at all?  If you're worried, let the first queueing record the worker
> > and trigger warning if someone tries to queue it anywhere else.  This
> > doesn't need to be full-on general like workqueue.  Let's make
> > reasonable trade-offs where possible.
> 
> I actually thought about this simplification as well. But then I am
> in doubts about the API. It would make sense to assign the worker
> when the work is being initialized and avoid the duplicate information
> when the work is being queued:
> 
> 	init_kthread_work(work, fn, worker);
> 	queue_work(work);
> 
> Or would you prefer to keep the API similar to workqueues even when
> it makes less sense here?
> 
> 
> In each case, we need a way to switch the worker if the old one
> is destroyed and a new one is started later. We would need
> something like:
> 
> 	reset_work(work, worker)
> or
> 	reinit_work(work, fn, worker)

I was too fast. We could set "work->worker = NULL" when the work
finishes and it is not pending. It means that it will be connected
to the particular worker only when used. Then we could keep the
workqueues-like API and do not need reset_work().

I am going to play with this. I feel that it might work.

Best Regards,
Petr
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