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

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Message-ID: <20140226052306.GD8264@linux.vnet.ibm.com>
Date:	Tue, 25 Feb 2014 21:23:06 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jeff Law <law@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Tue, Feb 25, 2014 at 08:32:38PM -0700, Jeff Law wrote:
> 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.

Agreed...

							Thanx, Paul

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