On Tue, 28 Jun 2011 23:29:52 +0200, Hans-Peter Diettrich
<drdiettri...@aol.com> wrote:
Vinzent Höfler schrieb:
No, it can't. "volatile" just ensures that accessing the variable
results in
actual memory accesses. That does not mean cache-coherence, so another
core may still see other (as in "older") values.
I dare to disagree. When reading a volatile variable requires a memory
(RAM) read in one core, it will require the same read in any other core,
and updates have to occur in write-through into the RAM.
Sorry, no. Memory access does not mean "RAM" here, it simply means that the
compiler is not allowed to keep the value in a register or collapse
multiple
writes to the same location into a single one. (Ages ago, this happened to
me
on a memory-mapped i8254, where the counter register requires two 8-bit
accesses to the same (memory-mapped) address and the compiler simply
removed
the first write).
So, memory access /might/ be the local cache of the core, unless the actual
memory region is write-through (because it's memory-mapped I/O, which was
the original intent of "volatile". See above.).
A difference could occur only, when the "memory" access may end up in
the cache instead of in RAM.
But that's precisely what would happen.
Question is, what makes one variable use read/write-through, while other
variables can be read from the cache, with lazy-write?
Synchronisation. Memory barriers. That's what they are for.
Is this a compiler requirement, which has to enforce read/write-through
for all volatile variables?
No. "volatile" (at least in C) does not mean that. It was never intended
for synchronisation between threads, cores, or even multiple processors.
This is a misconception probably stemming from the fact that it was used
to "synchronise" accesses to global variables within interrupt-handlers
and the like where this is totally valid as long as there's only one single
processor running all the code. The picture immediately changes if such an
interrupt-handler (e.g. "another thread") is executed on a different
processor/core/whatchamacallit.
But if so, which variables (class fields...) can ever be treated as
non-volatile, when they can be used from threads other than the main
thread?
Without explicit synchronisation? Actually, none.
Vinzent.
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