On Wed, 6 Dec 2006, David Howells wrote: > > Linus Torvalds <[EMAIL PROTECTED]> wrote: > > > (a) "volatile" on kernel data is basically always a bug, and you should > > use locking. > > But what about when you're building a lock? Actually, I suspect correct usage > of asm constraints and memory barriers trumps volatile anyway even there.
The word you look for is not "suspect". You _cannot_ build a lock using "volatile", unless your CPU is strictly in-order and has an in-order memory subsystem too (so, for example, while all ia64 implementations today are in-order, they do /not/ have an in-order memory subsystem). Only then could you do locking with volatile and some crazy Peterson's algorithm. I don't think any such CPU actually exists. Anyway, we've had this discussion before on linux-kernel, it really boils down to that "volatile" is basically never correct with the exception of flags that don't have any meaning and that you don't actually _care_ about the exact value (the low word of "jiffies" being the canonical example of something where "volatile" is actually fine, and where - as long as you can load it atomically - "volatile" really does make sense). Linus - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/