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


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