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

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Date:	Wed, 7 Oct 2015 07:24:46 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Petr Mladek <pmladek@...e.com>
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

Hello, Petr.

On Wed, Oct 07, 2015 at 11:21:30AM +0200, Petr Mladek wrote:
> Now, let's have one work: W, two workers: A, B, and try to queue
> the same work to the two workers at the same time:

It's a debug WARN condition to catch silly mistakes.  It can have
minor race conditions.

...
> Second, we still need the busy waiting for the pending timer callback.

Isn't that del_timer_sync()?

> Yes, we could set some flag so that the call back does not queue
> the work. But cancel_kthread_work_sync() still has to wait.
> It could not return if there is still some pending operation
> with the struct kthread_work. Otherwise, it never could
> be freed a safe way.
> 
> Also note that we still need the WORK_PENDING flag. Otherwise, we
> would not be able to detect the race when timer is removed but
> the callback has not run yet.

Yeah, just use a state field as I wrote before.

> Let me to repeat that using per-work and per-worker lock is not an
> option either. We would need some crazy hacks to avoid ABBA deadlocks.
> 
> 
> All in all, I would prefer to keep the original approach that is
> heavily inspired by the workqueues. I think that it is actually
> an advantage to reuse some working concept that reinventing wheels.

At each turn, you come up with non-issues and declare that it needs to
be full workqueue-like implementation but the issues you're raising
seem all rather irrelevant.  Can you please try to take a step back
and put some distance from the implementation details of workqueue?

Thanks.

-- 
tejun
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