Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

[Stephan Diestelhorst <stephan.diestelhorst@amd.com>]

On Wednesday 28 September 2011, 14:49:56 Linus Torvalds wrote:
> On Wed, Sep 28, 2011 at 11:08 AM, Stephan Diestelhorst
> <stephan.diestelhorst@amd.com> wrote:
> >
> > I must have missed the part when this turned into the propose-the-
> > craziest-way-that-this-still-works.contest :)
> 
> So doing it just with the "lock addb" probably works fine, but I have
> to say that I personally shudder at the "surround the locked addb by
> reads from the word, in order to approximate an atomic read of the
> upper bits".
> 
> Because what you get is not really an "atomic read of the upper bits",
> it's a "ok, we'll get the worst case of somebody modifying the upper
> bits at the same time".
> 
> Which certainly should *work*, but from a conceptual standpoint, isn't
> it just *much* nicer to say "we actually know *exactly* what the upper
> bits were".

Well, we really do NOT want atomicity here. What we really rather want
is sequentiality: free the lock, make the update visible, and THEN
check if someone has gone sleeping on it.

Atomicity only conveniently enforces that the three do not happen in a
different order (with the store becoming visible after the checking
load).

This does not have to be atomic, since spurious wakeups are not a
problem, in particular not with the FIFO-ness of ticket locks.

For that the fence, additional atomic etc. would be IMHO much cleaner
than the crazy overflow logic.

> But I don't care all *that* deeply. I do agree that the xaddw trick is
> pretty tricky. I just happen to think that it's actually *less* tricky
> than "read the upper bits separately and depend on subtle ordering
> issues with another writer that happens at the same time on another
> CPU".

Fair enough :)

Stephan
-- 
Stephan Diestelhorst, AMD Operating System Research Center
stephan.diestelhorst@amd.com
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