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

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Message-ID: <1409824374.4246.62.camel@pasglop>
Date:	Thu, 04 Sep 2014 19:52:54 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	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

On Thu, 2014-09-04 at 08:43 +0000, David Laight wrote:
> 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.

Yeah correct, alpha and bytes right ? Is there any other ? That's why I
suggested int.

> 	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.


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