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. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
