linux-kernel - Re: [PATCH RFC v2 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

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Date:	Tue, 27 Aug 2013 18:21:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Alexander Fyodorov <halcy@...dex.ru>,
	Waiman Long <waiman.long@...com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH RFC v2 1/2] qspinlock: Introducing a 4-byte queue
 spinlock implementation

On Tue, Aug 27, 2013 at 03:53:10PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 27, 2013 at 09:14:36AM -0400, Steven Rostedt wrote:
> 
> > I just had this conversation with Paul McKenney. Should there be a
> > smp_mb_after_spin_unlock()?
> 
> Depends on the benefits I suppose :-) Oleg and Linus did recently add
> smp_mb__before_spinlock();
> 
> > Although we blew it off as adding too many extensions to smp_mb(). But
> > it may be better than reimplementing something as complex as a lock.
> 
> Locks should be as light weight as possible and never implement anything
> heavier than the ACQUISITION / RELEASE barriers if at all possible. We
> should certainly not re-implement spinlocks just to get full barriers
> out of them, that's crazy.

An unlock followed by a lock needs to act like a full barrier, but there
is no requirement that a lock or unlock taken separately act like a
full barrier.

							Thanx, Paul

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