linux-kernel - avoiding run_workqueue() recursion

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Date:	Mon, 13 Jul 2009 21:45:05 -0700
From:	"Anirban Sinha" <ASinha@...gmasystems.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>
Cc:	<linux-kernel@...r.kernel.org>
Subject: avoiding run_workqueue() recursion

Hi Andrew:

I had a question about one of your previous commits: 

: commit 2355b70fd59cb5be7de2052a9edeee7afb7ff099
: Author: Lai Jiangshan <laijs@...fujitsu.com>
: Date:   Thu Apr 2 16:58:24 2009 -0700
:
:    workqueue: avoid recursion in run_workqueue()

http://git.kernel.org/linus/2355b70fd59cb5be7de2052a9edeee7afb7ff099 

I saw a few discussions on the mailing list around this. I also did see
your "I still don't know why I merged ..." comment on this. I have the
following observations. I am new in the kernel hacking world, so please
bear with me.

(a) I do agree that flushing the work queues from within run_workqueue()
is buggy in itself.

(b) I do also agree that recursive call to run_workqueue() is bad due to
the reasons cited in the commit log (even though I had a good laugh when
I saw the "morton gets to eat his hat" stuff :)).

(c) I am a little puzzled by the change the patch made. If we let the
call sleep on completion when keventd is itself running the
flush_workqueue(), are we not introducing a deadlock? If the thread that
is itself is responsible for walking the workqueue and dispatching the
work functions goes to sleep, who will wake it up?
In my honest opinion, I think we should simply return when (cwq->thread
== current) is true. I think in that condition,  it should be just a
nop.

Please let me know what you think.

cheers,

ani

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