linux-kernel - Re: [tip:core/locking] x86/smp: Move waiting on contended ticket lock out of line

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Date:	Fri, 01 Mar 2013 10:18:32 -0800
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.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,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [tip:core/locking] x86/smp: Move waiting on contended ticket
 lock out of line

On Fri, 2013-03-01 at 01:42 -0500, Rik van Riel wrote:
> On 02/28/2013 06:09 PM, Linus Torvalds wrote:
> 
> > So I almost think that *everything* there in the semaphore code could
> > be done under RCU. The actual spinlock doesn't seem to much matter, at
> > least for semaphores. The semaphore values themselves seem to be
> > protected by the atomic operations, but I might be wrong about that, I
> > didn't even check.
> 
> Checking try_atomic_semop and do_smart_update, it looks like neither
> is using atomic operations. That part of the semaphore code would
> still benefit from spinlocks.

Agreed.

> 
> The way the code handles a whole batch of semops all at once,
> potentially to multiple semaphores at once, and with the ability
> to undo all of the operations, it looks like the spinlock will
> still need to be per block of semaphores.
> 
> I guess the code may still benefit from Michel's locking code,
> after the permission stuff has been moved from under the spinlock.

How about splitting ipc_lock()/ipc_lock_control() in two calls: one to
obtain the ipc object (rcu_read_lock + idr_find), which can be called
when performing the permissions and security checks, and another to
obtain the ipcp->lock [q_]spinlock when necessary.


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