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

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Message-ID: <20140210114605.GI9987@twins.programming.kicks-ass.net>
Date:	Mon, 10 Feb 2014 12:46:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Torvald Riegel <triegel@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, Will Deacon <will.deacon@....com>,
	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>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.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 10, 2014 at 01:27:51AM +0100, Torvald Riegel wrote:
> > Initial state: x == y == 0
> > 
> > T1:	r1 = atomic_load_explicit(x, memory_order_relaxed);
> > 	atomic_store_explicit(42, y, memory_order_relaxed);
> > 	if (r1 != 42)
> > 		atomic_store_explicit(r1, y, memory_order_relaxed);
> > 
> > T2:	r2 = atomic_load_explicit(y, memory_order_relaxed);
> > 	atomic_store_explicit(r2, x, memory_order_relaxed);
> 
> Intuitively, this is wrong because this let's the program take a step
> the abstract machine wouldn't do.  This is different to the sequential
> code that Peter posted because it uses atomics, and thus one can't
> easily assume that the difference is not observable.

Yeah, my bad for not being familiar with the atrocious crap C11 made of
atomics :/

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