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

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Date:	Mon, 5 Oct 2015 12:07:58 +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 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)


Thanks for feedback.

Best Regards,
Petr
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