Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 2.6.27-rc4 released

On Thu, Aug 21, 2008 at 09:48:09AM -0700, Paul E. McKenney wrote:
On Thu, Aug 21, 2008 at 09:18:29AM -0700, Linus Torvalds wrote:


On Thu, 21 Aug 2008, Linus Torvalds wrote:

No, I haven't seen the drafts

Ok, I have looked at the draft now, and I don't think I was overly
pessimistic.

If I read it right, all the memory ordering operations are defined for
_single_ objects. So if you want to do the kernel kind of memory ordering
where you specify ordering requirements independently of the actual
accesses (perhaps because the accesses are in some helper function that
doesn't care, but then you want to "finalize" the thing by stating a
sequence point), it seems to be impossible with current drafts.

You are looking for atomic_fence() on page 1168 (1154 virtual) of the
most recent draft. The current semantics are not correct, but this is
being worked. And yes, it does currently have a variable associated with
it, but it acts as a bare fence nevertheless. There is a proposal to
drop the variable. As you said in a previous email, design by committee.

And the proposal for variable-free memory-ordering operations was voted
into the draft standard:

http://open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2731.html

Still not perfect, of course, but hopefully movement in the right
direction.

Thanx, Paul
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