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

[Linus Torvalds <torvalds@linux-foundation.org>]

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?

             Linus

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