On Thu, Sep 11, 2014 at 11:04:11AM +0100, One Thousand Gnomes wrote:

quoted

quoted

Is *that* what we are talking about?  I was added to this conversation
in the middle where it had already generalized, so I had no idea.

No, this is just what brought this craziness to my attention.

None of it is craziness. It's the real world leaking into the crazy
delusional world of sequential programming. Machines are going to get
more not less parallel.

Amen to that!!!

quoted

For example, byte- and short-sized circular buffers could not possibly
be safe either, when the head nears the tail.

Who has audited global storage and ensured that _every_ byte-sized write
doesn't happen to be adjacent to some other storage that may not happen
to be protected by the same (or any) lock?

Thats a meaningless question. Have you audited it all for correctness of
any other form. Have you mathematically verified the functionality as a
set of formal proofs ? If you can't prove its formally mathematically
functionally correct why are you worried about this ?

Alpha works, maybe it has a near theoretical race on that point. It's not
any worse than it was 15 years ago and nobody has really hit a problem
with it. So from that you can usefully infer that those buffer cases are
not proving a real problem.

Fair enough, I guess.

But Alpha's limitations were given as a reason to restrict
smp_store_release() and smp_load_acquire() from providing one-byte and
two-byte variants.  Of course, I am OK "probabilistically supporting"
pre-EV56 Alpha CPUs, but only if they don't get in the way of us doing
smp_store_release() and smp_load_acquire() on chars and shorts.  So if
pre-EV56 support has to go in order to allow smp_store_release() and
smp_load_acquire() on small data types, then pre-EV56 support simply
has to go.

Alternatively, one way to support this old hardware on a more
deterministic basis is to make the compiler use ll/sc sequences to do
byte and short accesses.  That would be fine as well.

						Thanx, Paul

The tty locks together on the other hand are asking to hit it, and the
problem you were trying to fix were the ones that need set_bit() to make
the guarantees.

Alan