Re: [RFC][PATCH] locking/mcs: Fix ordering for mcs_spin_lock()

[Peter Zijlstra]

On Mon, Feb 01, 2016 at 04:58:13PM +0000, Will Deacon wrote:
> Hi Peter,
> 
> On Mon, Feb 01, 2016 at 03:37:24PM +0100, Peter Zijlstra wrote:
> > Given the below patch; we've now got an unconditional full global
> > barrier in, does this make the MCS spinlock RCsc ?
> > 
> > The 'problem' is that this barrier can happen before we actually acquire
> > the lock. That is, if we hit arch_mcs_spin_lock_contended() _that_ will
> > be the acquire barrier and we end up with a SYNC in between unlock and
> > lock -- ie. not an smp_mb__after_unlock_lock() equivalent.
> 
> In which case, I don't think the lock will be RCsc with this change;
> you'd need an smp_mb__after_unlock_lock() after
> arch_mcs_spin_lock_contended(...) if you wanted the thing to be RCsc.

Right, I think it works for TSO, but in general it makes my head hurt.

> > This is non-critical because the MCS code isn't actually used and
> > mostly serves as documentation, a stepping stone to the more complex
> > things we've build on top of the idea.
> 
> Although I wonder how useful this is as a documentation aid now that we
> have the osq_lock.

So the OSQ thing is horribly complex, pure MCS is a nice step-stone.