linux-kernel - Re: [PATCH v9 06/12] kthread: Add kthread_drain

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Message-ID: <20160627143350.GA3313@pathway.suse.cz>
Date:	Mon, 27 Jun 2016 16:33:50 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	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>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 06/12] kthread: Add kthread_drain_worker()

Hi,

On Fri 2016-06-24 11:54:47, Tejun Heo wrote:
> On Fri, Jun 24, 2016 at 09:05:15AM +0200, Peter Zijlstra wrote:
> > > Given how rare that is 
> > 
> > Could you then not remove/rework these few cases for workqueue as well
> > and make that 'better' too?
> 
> Usage of draining is rare for workqueue but that still means several
> legitimate users.  With draining there, it's logical to use it during
> shutdown.  I don't think it makes sense to change it on workqueue
> side.
> 
> > > and the extra
> > > complexity of identifying self-requeueing cases, let's forget about
> > > draining and on destruction clear the worker pointer to block further
> > > queueing and then flush whatever is in flight.
> > 
> > You're talking about regular workqueues here?
> 
> No, kthread worker.  It's unlikely that kthread worker is gonna need
> chained draining especially given that most of its usages are gonna be
> conversions from raw kthread usages.  We won't lose much if anything
> by just ignoring draining and making the code simpler.

OK, so you suggest to do the following:

  1. Add a flag into struct kthread_worker that will prevent
     from further queuing.

  2. kthread_create_worker()/kthread_destroy_worker() will
     not longer dynamically allocate struct kthread_worker.
     They will just start/stop the kthread.


The result will be:

  a. User will not need the strict synchronization between
     the queue and create/destroy operations.

  b. We could get rid of drain_kthread_worker() because
     flush_kthread_worker() will be enough.


IMHO, the 1st change does not make sense without the 2nd one.
Otherwise, users could do an out-of-memory access when testing
the freed kthread_worker flag.

Do I get this correctly please?

Best Regards,
Petr