linux-kernel - Re: [RFC][PATCH 0/5] arch: atomic rework

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Date:	Mon, 24 Feb 2014 17:55:50 +0100 (CET)
From:	Michael Matz <matz@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Richard Biener <richard.guenther@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Torvald Riegel <triegel@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework

Hi,

On Mon, 24 Feb 2014, Linus Torvalds wrote:

> > To me that reads like
> >
> >   int i;
> >   int *q = &i;
> >   int **p = &q;
> >
> >   atomic_XXX (p, CONSUME);
> >
> > orders against accesses '*p', '**p', '*q' and 'i'.  Thus it seems they
> > want to say that it orders against aliased storage - but then go further
> > and include "indirectly through a chain of pointers"?!  Thus an
> > atomic read of a int * orders against any 'int' memory operation but
> > not against 'float' memory operations?
> 
> No, it's not about type at all, and the "chain of pointers" can be
> much more complex than that, since the "int *" can point to within an
> object that contains other things than just that "int" (the "int" can
> be part of a structure that then has pointers to other structures
> etc).

So, let me try to poke holes into your definition or increase my 
understanding :) .  You said "chain of pointers"(dereferences I assume), 
e.g. if p is result of consume load, then access to 
p->here->there->next->prev->stuff is supposed to be ordered with that load 
(or only when that last load/store itself is also an atomic load or 
store?).

So, what happens if the pointer deref chain is partly hidden in some 
functions:

A * adjustptr (B *ptr) { return &ptr->here->there->next; }
B * p = atomic_XXX (&somewhere, consume);
adjustptr(p)->prev->stuff = bla;

As far as I understood you, this whole ptrderef chain business would be 
only an optimization opportunity, right?  So if the compiler can't be sure 
how p is actually used (as in my function-using case, assume adjustptr is 
defined in another unit), then the consume load would simply be 
transformed into an acquire (or whatever, with some barrier I mean)?  Only 
_if_ the compiler sees all obvious uses of p (indirectly through pointer 
derefs) can it, yeah, do what with the consume load?

Ciao,
Michael.
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