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

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Message-ID: <CA+55aFyqLrj4d2TA+2aazRqXnbVsUvs0yaBL2D5rXF1G=Kiu_g@mail.gmail.com>
Date:	Mon, 17 Feb 2014 19:24:56 -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 Mon, Feb 17, 2014 at 7:00 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> One example that I learned about last week uses the branch-prediction
> hardware to validate value speculation.  And no, I am not at all a fan
> of value speculation, in case you were curious.

Heh. See the example I used in my reply to Alec Teal. It basically
broke the same dependency the same way.

Yes, value speculation of reads is simply wrong, the same way
speculative writes are simply wrong. The dependency chain matters, and
is meaningful, and breaking it is actively bad.

As far as I can tell, the intent is that you can't do value
speculation (except perhaps for the "relaxed", which quite frankly
sounds largely useless). But then I do get very very nervous when
people talk about "proving" certain values.

                Linus
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