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Date: Wed, 28 Aug 2013 10:19:37 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Alexander Fyodorov <halcy@yandex.ru>,
        Waiman Long <waiman.long@hp.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "Chandramouleeswaran, Aswin" <aswin@hp.com>,
        "Norton, Scott J" <scott.norton@hp.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH RFC v2 1/2] qspinlock: Introducing a 4-byte queue
 spinlock implementation
Message-ID: <20130828081937.GH10002@twins.programming.kicks-ass.net>
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On Tue, Aug 27, 2013 at 06:21:29PM -0700, Paul E. McKenney wrote:
> 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.

But that is already a property of the acquisition/release barrier.