linux-kernel - Re: [PATCH v5 00/44] ldisc patchset

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Date:	Thu, 14 Mar 2013 07:42:39 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Sasha Levin <levinsasha928@...il.com>,
	Dave Jones <davej@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Shawn Guo <shawn.guo@...aro.org>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH v5 00/44] ldisc patchset

On Thu, 2013-03-14 at 00:25 -0700, Michel Lespinasse wrote:
> On Wed, Mar 13, 2013 at 6:12 PM, Peter Hurley <peter@...leysoftware.com> wrote:
> > On Wed, 2013-03-13 at 04:36 -0700, Michel Lespinasse wrote:
> >> Have you considered building your ldlock based on lib/rwsem-spinlock.c
> >> instead ? i.e. having an internal spinlock to protect the ldisc
> >> reference count and the reader and writer queues. This would seem much
> >> simpler get right. The downside would be that a spinlock would be
> >> taken for a short time whenever an ldisc reference is taken or
> >> released. I don't expect that the internal spinlock would get
> >> significant contention ?
> >
> > That would have been too easy :)
> >
> > TBH, I hadn't considered it until I was most the way through a working
> > atomic version. I had already split the reader/writer wait lists. And
> > figured out how to always use the wait bias for every waiting reader and
> > writer -- rather than the rwsem way of testing for an empty list --
> > which made the timeout handling easier.
> >
> > At the time, the only thing that I was still struggling with was
> > recursion, and the spinlock flavor wasn't going to fix that. So I just
> > kept with the atomic flavor.
> 
> Its not too late to run away from it and preserve your sanity (as well
> as that of the next person working on the tty layer :)

The long-term plan is to migrate it to lib so it won't be a maintenance
burden to tty.

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