Subj : Re: lockless low-overhead 'pipes' (w/semaphores)
To   : comp.programming.threads
From : lindahlb
Date : Tue Apr 19 2005 09:14 pm

> I depends upon your assumed memory model. If, for example, you're
just
> assuming a POSIX-compliant environment, there's no guarantee the
consumer
> will read the data the producer wrote (instead of stale data stuck in
some
> cache somewhere), since the consumer doesn't hold the same lock the
producer
> held.

Ok, so in the problem you iterate, the producer stores some data, which
invariably ends up staying in a processor's local cache (say L1 or L2),
and when the consumer, executing on another processor, would attempt to
access the data, it wouldn't see the cache data located on the other
processor? Don't today's SMP architectures guarantee visibility in
situations like this (i.e. cache collision)?

If they do - the data is stored and THEN 'len' is updated. If this
order is preserved, then how could the consumer see the updated 'len'
before the 'data' has reached memory?

If you're talking about out-of-order reads/writes, both compiler and
processor level - the atomic_* functions imply a memory barrier (lock
prefix for i486+), so thats a non-issue, right? Or does the lock prefix
not enforce order and I need a seperate instruction to prevent
store-store (producer) and load-store (consumer) barriers?

Also, if the lock prefix doesn't work that way I thought it did, are
there any other locations that need types of memory barriers that I may
be missing?

However, if the problem you mention exists NOT because of out-of-order
stores/loads, and that the 'len' could reach memory, but the 'data' is
still suck in the cache, even after a 'lock' prefix, then I don't see
how any program executing on multiple processors could guarantee
coherency between data on seperate processors.  If there's no way to
gaurantee cache coherency, then synchronization would be near
impossible, no?

I thought I understood this quite well, but clearly I must be missing
something (unless I am not enforcing an order to 'len' and 'data' via
the lock prefix).

Could you please elaborate? I really would appreciate the help, thanks.

.