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

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Date:	Wed, 19 Feb 2014 07:23:27 -0800 (PST)
From:	David Lang <david@...g.hm>
To:	Torvald Riegel <triegel@...hat.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alec Teal <a.teal@...wick.ac.uk>,
	Paul McKenney <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>,
	"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 Tue, 18 Feb 2014, Torvald Riegel wrote:

> On Tue, 2014-02-18 at 22:40 +0100, Peter Zijlstra wrote:
>> On Tue, Feb 18, 2014 at 10:21:56PM +0100, Torvald Riegel wrote:
>>> Well, that's how atomics that aren't volatile are defined in the
>>> standard.  I can see that you want something else too, but that doesn't
>>> mean that the other thing is broken.
>>
>> Well that other thing depends on being able to see the entire program at
>> compile time. PaulMck already listed various ways in which this is
>> not feasible even for normal userspace code.
>>
>> In particular; DSOs and JITs were mentioned.
>
> No it doesn't depend on whole-program analysis being possible.  Because
> if it isn't, then a correct compiler will just not do certain
> optimizations simply because it can't prove properties required for the
> optimization to hold.  With the exception of access to objects via magic
> numbers (e.g., fixed and known addresses (see my reply to Paul), which
> are outside of the semantics specified in the standard), I don't see a
> correctness problem here.

Are you really sure that the compiler can figure out every possible thing that a 
loadable module or JITed code can access? That seems like a pretty strong claim.

David Lang
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