linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Message-Id: <e1aed5ce66e392cd290f82637b2176f2@kernel.crashing.org>
Date:	Tue, 21 Aug 2007 16:59:08 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Chris Snook <csnook@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, heiko.carstens@...ibm.com,
	ilpo.jarvinen@...sinki.fi, horms@...ge.net.au,
	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	rpjday@...dspring.com, netdev@...r.kernel.org, ak@...e.de,
	piggin@...erone.com.au, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, cfriesen@...tel.com,
	jesper.juhl@...il.com, linux-arch@...r.kernel.org,
	paulus@...ba.org, herbert@...dor.apana.org.au,
	satyam@...radead.org, clameter@....com, stefanr@...6.in-berlin.de,
	schwidefsky@...ibm.com, zlynx@....org, wensong@...ux-vs.org,
	wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

> At some point in the future, barrier() will be universally regarded as 
> a hammer too big for most purposes.  Whether or not removing it now

You can't just remove it, it is needed in some places; you want to
replace it in most places with a more fine-grained "compiler barrier",
I presume?

> constitutes premature optimization is arguable, but I think we should 
> allow such optimization to happen (or not happen) in 
> architecture-dependent code, and provide a consistent API that doesn't 
> require the use of such things in arch-independent code where it might 
> turn into a totally superfluous performance killer depending on what 
> hardware it gets compiled for.

Explicit barrier()s won't be too hard to replace -- but what to do
about the implicit barrier()s in rmb() etc. etc. -- *those* will be
hard to get rid of, if only because it is hard enough to teach driver
authors about how to use those primitives *already*.  It is far from
clear what a good interface like that would look like, anyway.

Probably we should first start experimenting with a forget()-style
micro-barrier (but please, find a better name), and see if a nice
usage pattern shows up that can be turned into an API.


Segher

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