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

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Message-ID: <20080924000136.GA12897@linux.vnet.ibm.com>
Date:	Tue, 23 Sep 2008 17:01:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	jmerkey@...fmountaingroup.com, linux-kernel@...r.kernel.org,
	Nick Piggin <nickpiggin@...oo.com.au>,
	David Howells <dhowells@...hat.com>
Subject: 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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