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. _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev