linux-kernel - Re: [PATCH 03/18] tty: Simplify tty buffer/ldisc interface with helper function

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Message-ID: <5149AFD0.8020604@izyk.ru>
Date:	Wed, 20 Mar 2013 16:47:12 +0400
From:	Ilya Zykov <linux@...k.ru>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, Min Zhang <mzhang@...sta.com>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/18] tty: Simplify tty buffer/ldisc interface with helper
 function

>> I have little question about flush_to_ldisc().
>> Does can it be multithreaded?
>>
>> I think yes, because on SMP schedule_work() can work on different CPU paralleled.
> 
> Yes, the same work item can now run in parallel on SMP since Tejun Heo
> re-did the workqueue implementation on 2.6.36 [Stefan Richter, the
> firewire maintainer, recently explained this history to me].

About multi threaded delayed works:

In many cases tty layer needs single threaded delayed work for each tty instance.
I propose discussion about create for this purpose (tty layer)'s workqueue with WQ_NON_REENTRANT flag.
And use it instead common schedule_work()'s workqueue - system_wq.
I don't know how expensive(for system resource and CPU) it can be,
but for tty layer, it can be very useful.

> 
>> What do you think about this race condition?
>> https://lkml.org/lkml/2011/11/7/98
> 
> Yes, that is a possible race condition that could lead to some nasty
> results. Good find.
> 
> If you want, I could bring that patch into this patchset or you could
> re-submit that patch to Greg and I could rebase this patchset on top of
> that.

Peter,
Please, do it anywhere you consider possible,
I can't do it myself now.

Thank you,
Ilya.

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