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

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Message-ID: <530D6056.40305@redhat.com>
Date:	Tue, 25 Feb 2014 20:32:38 -0700
From:	Jeff Law <law@...hat.com>
To:	paulmck@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
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 02/25/14 17:15, Paul E. McKenney wrote:
>> I have for the last several years been 100% convinced that the Intel
>> memory ordering is the right thing, and that people who like weak
>> memory ordering are wrong and should try to avoid reproducing if at
>> all possible. But given that we have memory orderings like power and
>> ARM, I don't actually see a sane way to get a good strong ordering.
>> You can teach compilers about cases like the above when they actually
>> see all the code and they could poison the value chain etc. But it
>> would be fairly painful, and once you cross object files (or even just
>> functions in the same compilation unit, for that matter), it goes from
>> painful to just "ridiculously not worth it".
>
> And I have indeed seen a post or two from you favoring stronger memory
> ordering over the past few years.  ;-)
I couldn't agree more.

>
> Are ARM and Power really the bad boys here?  Or are they instead playing
> the role of the canary in the coal mine?
That's a question I've been struggling with recently as well.  I suspect 
they (arm, power) are going to be the outliers rather than the canary. 
While the weaker model may give them some advantages WRT scalability, I 
don't think it'll ultimately be enough to overcome the difficulty in 
writing correct low level code for them.

Regardless, they're here and we have to deal with them.


Jeff
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