linux-kernel - Re: [RFC PATCH 1/2] ipc: introduce obtaining a lockless ipc object

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Date:	Sun, 03 Mar 2013 06:53:10 +0100
From:	Mike Galbraith <bitbucket@...ine.de>
To:	Rik van Riel <riel@...hat.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	"Low, Jason" <jason.low2@...com>,
	linux-tip-commits@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>, aquini@...hat.com,
	Ingo Molnar <mingo@...nel.org>,
	Larry Woodman <lwoodman@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 1/2] ipc: introduce obtaining a lockless ipc object

On Sat, 2013-03-02 at 21:18 -0500, Rik van Riel wrote: 
> On 03/01/2013 11:32 PM, Michel Lespinasse wrote:
> 
> > I think it may be nicer to take the rcu read lock at the call site
> > rather than in ipc_obtain_object(), to make the rcu read lock/unlock
> > sites pair up more nicely. Either that or make an inline
> > ipc_release_object function that pairs up with ipc_obtain_object() and
> > just does an rcu_read_unlock().
> 
> I started on a patch series to untangle the IPC locking, so
> it will be a little more readable, and easier to maintain.
> 
> It is a slower approach than Davidlohr's, as in, it will take
> a little longer to put a patch series together, but I hope it
> will be easier to debug...
> 
> I hope to post a first iteration of the series by the middle
> of next week.

Goody, I'll be watching out for it.  I have a big box rt user who uses
semaphores in their very tight constraint application.  While they're
using them carefully, I saw a trace where contention cost vs jitter
budget was a bit too high for comfort, and semctl/semop collision.

-Mike

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