linux-kernel - Re: bit fields && data tearing

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Message-ID: <5408C0AB.6050801@hurleysoftware.com>
Date:	Thu, 04 Sep 2014 15:42:35 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
CC:	Jakub Jelinek <jakub@...hat.com>,
	Mikael Pettersson <mikpelinux@...il.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Richard Henderson <rth@...ddle.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Miroslav Franc <mfranc@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
	linux-ia64@...r.kernel.org
Subject: Re: bit fields && data tearing

On 09/04/2014 12:50 PM, One Thousand Gnomes wrote:
>> Besides updating the documentation, it may make sense to do something
>> arch-specific. Just bumping out storage on arches that don't need it
>> seems wasteful, as does generating bus locks on arches that don't need it.
>> Unfortunately, the code churn looks unavoidable.
> 
> The arch specific is pretty much set_bit and friends. Bus locks on a
> locally owned cache line should not be very expensive on anything vaguely
> modern, while uniprocessor boxes usually only have to generate set_bit
> as a single instruction so it is interrupt safe.

Or we could give up on the Alpha.

It's not just the non-atomic bytes; we could do away with the
read_barrier_depends() which hardly any code gets correctly anyway.

Regards,
Peter Hurley

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