>> On Thu, 2008-08-21 at 23:37 +1000, Nick Piggin wrote:
>> > On Thursday 21 August 2008 22:26,
jmerkey@wolfmountaingroup.com wrote:
>> >
>> > > I used the smp_wmb() functions. I noted a couple of things. a)
>> some of
>> > > these macros just emit __asm__ __volatile__ into the code so why not
>> just
>> > > say "volatile" to begin with
>> >
>> > It is not the same as volatile type. What it does is tell the compiler
>> > to clobber all registers or temporaries. This something pretty well
>> > defined and hard to get wrong compared to volatile type.
>>
>> Right, asm volatile () means that the asm may not be discarted. Very
>> different from the volatile type qualifier.
>>
>> > > b) smp_wmb() in some cases worked and in
>> > > other cases jut optimized away the global reference.
>> >
>> > Linux barriers aren't going to force a load to be emitted, if it can
>> be
>> > optimized away. If it optimized away a store, then I'd like to see a
>> > test case.
>>
>> Not sure - I think all barrier clobber the full register and memory set.
>> So if you access a variable after a barrier it will have to issue a
>> load.
>
> Here is one example (which might or might not be what Nick had in mind):
>
> extern int v;
>
> void foo(void)
> {
> do_something_with(v);
> barrier();
> do_something_else_with(v - v);
> }
>
> The second set of loads from v can be optimized away unless v is
> declared volatile. In contrast:
>
> void bar(void)
> {
> do_something_with(v);
> barrier();
> do_something_else_with(v);
> }
>
> Here the compiler must refetch v after the barrier.
>
>> Are we talking about different things?
>>
>> > > c) I can go back and
>> > > break the code again by inserting them and building broken assembler
>> d) I
>> > > ave been doing hardware and software design since the early 1980;s,
>> I
>> > > invented SMP affinity scheduling, and yes, I understand barriers and
>> this
>> > > concept of instruction score-boarding and optimization very well --
>> its
>> > > not an excuse for a busted C compiler.
>> >
>> > The point is not whether it is possible to work with volatile types,
>> but
>> > that we tend not to use them in Linux to deal with concurrency.
>> >
>> > Also, barriers seem to work fine for everybody else, so I think it is
>> > likely you either aren't using them correctly, or have other bugs in
>> the
>> > code.
>>
>> Well, there is of course the third option, which is what Jeff claims,
>> that gcc is broken. But in that case we should have more problems
>> elsewhere too.
>
> Given the amount of code in gcc, we can reasonably assume that some
> aspects of it are broken. Whether that presumed breakage is affecting
> Jeff is another question altogether. ;-)
>
> Thanx, Paul
>
