On Friday 17 August 2007 17:48, Linus Torvalds wrote:

On Fri, 17 Aug 2007, Nick Piggin wrote:

quoted

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