Re: [PATCH v6 4/5] MCS Lock: Barrier corrections

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

On Wed, Nov 27, 2013 at 10:16:13AM +0000, Will Deacon wrote:
> On Tue, Nov 26, 2013 at 10:51:36PM +0000, Paul E. McKenney wrote:
> > On Tue, Nov 26, 2013 at 11:32:25AM -0800, Linus Torvalds wrote:
> > > On Tue, Nov 26, 2013 at 11:20 AM, Paul E. McKenney
> > > <paulmck@linux.vnet.ibm.com> wrote:
> > > >
> > > > There are several places in RCU that assume unlock+lock is a full
> > > > memory barrier, but I would be more than happy to fix them up given
> > > > an smp_mb__after_spinlock() and an smp_mb__before_spinunlock(), or
> > > > something similar.
> > > 
> > > A "before_spinunlock" would actually be expensive on x86.
> > 
> > Good point, on x86 the typical non-queued spin-lock acquisition path
> > has an atomic operation with full memory barrier in any case.  I believe
> > that this is the case for the other TSO architectures.  For the non-TSO
> > architectures:
> > 
> > o	ARM has an smp_mb() during lock acquisition, so after_spinlock()
> > 	can be a no-op for them.
> 
> Ok, but what about arm64? We use acquire for lock() and release for
> unlock(), so in Linus' example:

Right, I did forget the arm vs. arm64 split!

>     write A;
>     spin_lock()
>     mb__after_spinlock();
>     read B
> 
> Then A could very well be reordered after B if mb__after_spinlock() is a nop.
> Making that a full barrier kind of defeats the point of using acquire in the
> first place...

The trick is that you don't have mb__after_spinlock() unless you need the
ordering, which we expect in a small minority of the lock acquisitions.
So you would normally get the benefit of acquire/release efficiency.

> It's one thing ordering unlock -> lock, but another getting those two to
> behave as full barriers for any arbitrary memory accesses.

And in fact the unlock+lock barrier is all that RCU needs.  I guess the
question is whether it is worth having two flavors of __after_spinlock(),
one that is a full barrier with just the lock, and another that is
only guaranteed to be a full barrier with unlock+lock.

								Thanx, Paul

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