linux-kernel - Re: [PATCH] x86: Remove readq()/writeq() on 32-bit

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Message-ID: <4A0B30D0.4060806@garzik.org>
Date:	Wed, 13 May 2009 16:42:56 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Hitoshi Mitake <h.mitake@...il.com>
CC:	Roland Dreier <rdreier@...co.com>, Ingo Molnar <mingo@...e.hu>,
	David Miller <davem@...emloft.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>, hpa@...or.com,
	tglx@...utronix.de, rpjday@...shcourse.ca,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: Remove readq()/writeq() on 32-bit

Hitoshi Mitake wrote:
> I think it's good time to decide making all architectures
> which have readq/writeq provide HAVE_READQ/HAVE_WRITEQ or not.
> 
> Adding HAVE_READQ/HAVE_WRITEQ to Kconfig of architectures needs
> agreement of all maintainers of these.
> 
> But, David Miller, maintainer of SPARC architecture, acked Roland's patch
> because of the possibility of bugs non-atomicity of readq/writeq of
> x86-32 will cause.
> 
> And, Jeff Garzik said that he saw zero justification for API removal.
> 
> Which way should we choose?
> Remove readq/writeq from x86-32?
> Or add HAVE... to all architectures with readq/writeq?

To repeat what has already been stated, each case was re-evaluated: 
http://marc.info/?l=linux-kernel&m=124103527326835&w=2

Roland's patch was acked, apparently, _in spite of_ the commonly 
accepted readq() definition already being in use!

Thusfar, I see two things:

(1) years of history has shown that non-atomic readq/writeq on 32-bit 
platforms has been sufficient, based on testing and experience.  In 
fact, in niu's case, a common readq/writeq would have PREVENTED a bug.

(2) unspecified fears continue to linger about non-atomicity

We should not base decisions on fear, particularly when the weight of 
evidence and experience points in the other direction.

	Jeff



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