bit fields && data tearing

[David Laight]

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.