On Friday 17 August 2007 17:48, Linus Torvalds wrote:
>
> On Fri, 17 Aug 2007, Nick Piggin wrote:
> >
> > That's not obviously just taste to me. Not when the primitive has many
> > (perhaps, the majority) of uses that do not require said barriers. And
> > this is not solely about the code generation (which, as Paul says, is
> > relatively minor even on x86). I prefer people to think explicitly
> > about barriers in their lockless code.
>
> Indeed.
>
> I think the important issues are:
>
> - "volatile" itself is simply a badly/weakly defined issue. The semantics
> of it as far as the compiler is concerned are really not very good, and
> in practice tends to boil down to "I will generate so bad code that
> nobody can accuse me of optimizing anything away".
>
> - "volatile" - regardless of how well or badly defined it is - is purely
> a compiler thing. It has absolutely no meaning for the CPU itself, so
> it at no point implies any CPU barriers. As a result, even if the
> compiler generates crap code and doesn't re-order anything, there's
> nothing that says what the CPU will do.
>
> - in other words, the *only* possible meaning for "volatile" is a purely
> single-CPU meaning. And if you only have a single CPU involved in the
> process, the "volatile" is by definition pointless (because even
> without a volatile, the compiler is required to make the C code appear
> consistent as far as a single CPU is concerned).
>
> So, let's take the example *buggy* code where we use "volatile" to wait
> for other CPU's:
>
> atomic_set(&var, 0);
> while (!atomic_read(&var))
> /* nothing */;
>
>
> which generates an endless loop if we don't have atomic_read() imply
> volatile.
>
> The point here is that it's buggy whether the volatile is there or not!
> Exactly because the user expects multi-processing behaviour, but
> "volatile" doesn't actually give any real guarantees about it. Another CPU
> may have done:
>
> external_ptr = kmalloc(..);
> /* Setup is now complete, inform the waiter */
> atomic_inc(&var);
>
> but the fact is, since the other CPU isn't serialized in any way, the
> "while-loop" (even in the presense of "volatile") doesn't actually work
> right! Whatever the "atomic_read()" was waiting for may not have
> completed, because we have no barriers!
Why is all this fixation on "volatile"? I don't think
people want "volatile" keyword per se, they want atomic_read(&x) to
_always_ compile into an memory-accessing instruction, not register access.
--
vda
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