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

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Date:	Mon, 24 Feb 2014 17:27:26 +0100
From:	Richard Biener <richard.guenther@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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

On Mon, Feb 24, 2014 at 4:57 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Sun, Feb 23, 2014 at 11:31 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> Let me think about it some more, but my gut feel is that just tweaking
>> the definition of what "ordered" means is sufficient.
>>
>> So to go back to the suggested ordering rules (ignoring the "restrict"
>> part, which is just to clarify that ordering through other means to
>> get to the object doesn't matter), I suggested:
>>
>>  "the consume ordering guarantees the ordering between that
>>   atomic read and the accesses to the object that the pointer
>>   points to"
>>
>> and I think the solution is to just say that this ordering acts as a
>> fence. It doesn't say exactly *where* the fence is, but it says that
>> there is *some* fence between the load of the pointer and any/all
>> accesses to the object through that pointer.
>
> I'm wrong. That doesn't work. At all. There is no ordering except
> through the pointer chain.
>
> So I think saying just that, and nothing else (no magic fences, no
> nothing) is the right thing:
>
>  "the consume ordering guarantees the ordering between that
>   atomic read and the accesses to the object that the pointer
>   points to directly or indirectly through a chain of pointers"

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?

Eh ...

Just jumping in to throw in my weird-2-cents.

Richard.
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