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

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

On Tue, Nov 26, 2013 at 03:58:11PM -0800, Linus Torvalds wrote:
> On Tue, Nov 26, 2013 at 2:51 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> >
> > Good points, and after_spinlock() works for me from an RCU perspective.
> 
> Note that there's still a semantic question about exactly what that
> "after_spinlock()" is: would it be a memory barrier *only* for the CPU
> that actually does the spinlock? Or is it that "third CPU" order?
> 
> IOW, it would stil not necessarily make your "unlock+lock" (on
> different CPU's) be an actual barrier as far as a third CPU was
> concerned, because you could still have the "unlock happened after
> contention was going on, so the final unlock only released the MCS
> waiter, and there was no barrier".
> 
> See what I'm saying? We could guarantee that if somebody does
> 
>     write A;
>     spin_lock()
>     mb__after_spinlock();
>     read B
> 
> then the "write A" -> "read B" would be ordered. That's one thing.
> 
> But your
> 
>  -  CPU 1:
> 
>     write A
>     spin_unlock()
> 
>  - CPU 2
> 
>     spin_lock()
>     mb__after_spinlock();
>     read B
> 
> ordering as far as a *third* CPU is concerned is a whole different
> thing again, and wouldn't be at all the same thing.
> 
> Is it really that cross-CPU ordering you care about?

Cross-CPU ordering.  I have to guarantee the grace period across all
CPUs, and I currently rely on a series of lock acquisitions to provide
that ordering.  On the other hand, I only rely on unlock+lock pairs,
so that I don't need any particular lock or unlock operation to be
a full barrier in and of itself.

If that turns out to be problematic, I could of course insert smp_mb()s
everywhere, but they would be redundant on most architectures.

							Thanx, Paul

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>