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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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