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

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Date:	Sun, 23 Feb 2014 21:25:55 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	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 Sun, Feb 23, 2014 at 8:59 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Sun, Feb 23, 2014 at 05:35:28PM -0800, Linus Torvalds wrote:
>>
>> But "q = p->next" is ordered by how something can alias "p->next", not by 'q'!
>>
>> There is no need to restrict anything but 'p' for all of this to work.
>
> I cannot say I understand this last sentence right new from the viewpoint
> of the standard, but suspending disbelief for the moment...

So 'p' is what comes from that consuming load that returns a
'restrict' pointer. That doesn't affect 'q' in any way.

But the act of initializing 'q' by dereferencing p (in "p->next") is -
by virtue of the restrict - something that the compiler can see cannot
alias with anything else, so the compiler could re-order other memory
accesses freely around it, if you see what I mean.

Modulo all the *other* ordering guarantees, of course. So other
atomics and volatiles etc may have their own rules, quite apart from
any aliasing issues.

> Understood -- in this variant, you are taking the marking from the
> fact that there was an assignment from a memory_order_consume load
> rather than from a keyword on the assigned-to variable's declaration.

Yes, and to me, it's really just a legalistic trick to make it clear
that any *other* pointer that  happens to point to the same object
cannot be dereferenced within scope of the result of the
atomic_read(mo_consume), at least not if you expect to get the memory
ordering semantics. You can do it, but then you violate the guarantee
of the restrict, and you get what you get - a potential non-ordering.

So if somebody just immediately assigns the value to a normal
(non-restrict) pointer nothing *really* changes. It's just there to
describe the guarantees.

              Linus
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