linux-kernel - Re: [RFC v2 03/18] kthread: Add drain_kthread

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Message-ID: <20150922182659.GB17659@mtj.duckdns.org>
Date:	Tue, 22 Sep 2015 14:26:59 -0400
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 03/18] kthread: Add drain_kthread_worker()

On Mon, Sep 21, 2015 at 03:03:44PM +0200, Petr Mladek wrote:
> flush_kthread_worker() returns when the currently queued works are proceed.
								     ^
								   processed?

> But some other works might have been queued in the meantime.
...
> +/**
> + * drain_kthread_worker - drain a kthread worker
> + * @worker: worker to be drained
> + *
> + * Wait until there is none work queued for the given kthread worker.
                          ^
                          no

> + * Only currently running work on @worker can queue further work items
                                             ^^^^^^^^^
				 should be queueing is prolly more accurate

> + * on it.  @worker is flushed repeatedly until it becomes empty.
> + * The number of flushing is determined by the depth of chaining
> + * and should be relatively short.  Whine if it takes too long.
> + *
> + * The caller is responsible for blocking all existing works
> + * from an infinite re-queuing!
           
The caller is responsible for preventing the existing work items from
requeueing themselves indefinitely.

> + *
> + * Also the caller is responsible for blocking all the kthread
> + * worker users from queuing any new work. It is especially
> + * important if the queue has to stay empty once this function
> + * finishes.

The last sentence reads a bit weird to me.  New work items just aren't
allowed while draining.  It isn't "especially important" for certain
cases.  It's just buggy otherwise.

Thanks.

-- 
tejun
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