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Date: Tue, 27 Aug 2013 18:21:29 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>, Alexander Fyodorov <halcy@yandex.ru>,
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        "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: <20130828012128.GT3871@linux.vnet.ibm.com>
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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