Satyam Sharma wrote:
> On Wed, 15 Aug 2007, Stefan Richter wrote:
>> Doesn't "atomic WRT all processors" require volatility?
>
> No, it definitely doesn't. Why should it?
>
> "Atomic w.r.t. all processors" is just your normal, simple "atomicity"
> for SMP systems (ensure that that object is modified / set / replaced
> in main memory atomically) and has nothing to do with "volatile"
> behaviour.
>
> "Volatile behaviour" itself isn't consistently defined (at least
> definitely not consistently implemented in various gcc versions across
> platforms), but it is /expected/ to mean something like: "ensure that
> every such access actually goes all the way to memory, and is not
> re-ordered w.r.t. to other accesses, as far as the compiler can take
> care of these". The last "as far as compiler can take care" disclaimer
> comes about due to CPUs doing their own re-ordering nowadays.
>
> For example (say on i386):
[...]
> In (A) the compiler optimized "a = 10;" away, but the actual store
> of the final value "20" to "a" was still "atomic". (B) and (C) also
> exhibit "volatile" behaviour apart from the "atomicity".
>
> But as others replied, it seems some callers out there depend upon
> atomic ops exhibiting "volatile" behaviour as well, so that answers
> my initial question, actually. I haven't looked at the code Paul
> pointed me at, but I wonder if that "forget(x)" macro would help
> those cases. I'd wish to avoid the "volatile" primitive, personally.
So, looking at load instead of store, understand I correctly that in
your opinion
int b;
b = atomic_read(&a);
if (b)
do_something_time_consuming();
b = atomic_read(&a);
if (b)
do_something_more();
should be changed to explicitly forget(&a) after
do_something_time_consuming?
If so, how about the following:
static inline void A(atomic_t *a)
{
int b = atomic_read(a);
if (b)
do_something_time_consuming();
}
static inline void B(atomic_t *a)
{
int b = atomic_read(a);
if (b)
do_something_more();
}
static void C(atomic_t *a)
{
A(a);
B(b);
}
Would this need forget(a) after A(a)?
(Is the latter actually answered in C99 or is it compiler-dependent?)
--
Stefan Richter
-=====-=-=== =--- -====
http://arcgraph.de/sr/
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