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

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Date:	Thu, 4 Sep 2014 08:43:13 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Benjamin Herrenschmidt' <benh@...nel.crashing.org>,
	Peter Hurley <peter@...leysoftware.com>
CC:	Jakub Jelinek <jakub@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Miroslav Franc <mfranc@...hat.com>,
	"Richard Henderson" <rth@...ddle.net>
Subject: RE: bit fields && data tearing

From:  Benjamin Herrenschmidt
> 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).

That isn't true on all architectures for items smaller than a machine word.
At least one has to do rmw for byte accesses.

	David

> 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.
> 
> Another option is to use the atomic bitops and make these bits in a
> bitmask but that is probably unnecessary if you have locks already.
> 
> Cheers,
> Ben.