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

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Message-ID: <20140904090952.GW17454@tucnak.redhat.com>
Date:	Thu, 4 Sep 2014 11:09:52 +0200
From:	Jakub Jelinek <jakub@...hat.com>
To:	Mikael Pettersson <mikpelinux@...il.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Hurley <peter@...leysoftware.com>,
	"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 Thu, Sep 04, 2014 at 10:57:40AM +0200, Mikael Pettersson wrote:
> Benjamin Herrenschmidt writes:
>  > On Wed, 2014-09-03 at 18:51 -0400, Peter Hurley wrote:
>  > 
>  > > Apologies for hijacking this thread but I need to extend this discussion
>  > > somewhat regarding what a compiler might do with adjacent fields in a structure.
>  > > 
>  > > The tty subsystem defines a large aggregate structure, struct tty_struct.
>  > > Importantly, several different locks apply to different fields within that
>  > > structure; ie., a specific spinlock will be claimed before updating or accessing
>  > > certain fields while a different spinlock will be claimed before updating or
>  > > accessing certain _adjacent_ fields.
>  > > 
>  > > What is necessary and sufficient to prevent accidental false-sharing?
>  > > The patch below was flagged as insufficient on ia64, and possibly ARM.
>  > 
>  > We expect native aligned scalar types to be accessed atomically (the
>  > read/modify/write of a larger quantity that gcc does on some bitfield
>  > cases has been flagged as a gcc bug, but shouldn't happen on normal
>  > scalar types).
>  > 
>  > I am not 100% certain of "bool" here, I assume it's treated as a normal
>  > scalar and thus atomic but if unsure, you can always use int.
> 
> Please use an aligned int or long.  Some machines cannot do atomic
> accesses on sub-int/long quantities, so 'bool' may cause unexpected
> rmw cycles on adjacent fields.

Yeah, at least pre-EV56 Alpha performs rmw cycles on char/short accesses
and thus those are not atomic.

	Jakub
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