Re: bit fields && data tearing

Jakub Jelinek
2014-09-04T09:09:52Z

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