On Thu, 2008-08-21 at 23:37 +1000, Nick Piggin wrote:
quoted text
> 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.

quoted text
> > 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.

Are we talking about different things?

quoted text
> > 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.


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