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

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Message-ID: <CA+55aFyYtMm1_UsPVw8mi7o2iH_--1wLvxPUt5Z9mAEOGMgXdQ@mail.gmail.com>
Date:	Mon, 24 Feb 2014 08:37:43 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Richard Biener <richard.guenther@...il.com>
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 8:27 AM, Richard Biener
<richard.guenther@...il.com> 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 in your example,

    ptr = atomic_read(p, CONSUME);

would indeed order against the subsequent access of the chain through
*that* pointer (the whole "restrict" thing that I left out as a
separate thing, which was probably a mistake), but certainly not
against any integer pointer, and certainly not against any aliasing
pointer chains.

So yes, the atomic_read() would be ordered wrt '*ptr' (getting 'q')
_and_ '**ptr' (getting 'i'), but nothing else - including just the
aliasing access of dereferencing 'i' directly.

            Linus
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