* Linus Torvalds <[EMAIL PROTECTED]> wrote: > With the sequence counters, the situation is more complex: > > CPU #0 CPU #1 > > A (= code before the spinlock) > > lock xadd mem (serializing instruction) > > B (= code afte xadd, but not inside lock) > > lock release > > cmp head, tail > > C (= code inside the lock) > > Now, B is basically the empty set, but that's not the issue I worry > about. The thing is, I can guarantee by the Intel memory ordering > rules that neither B nor C will ever have memops that leak past the > "xadd", but I'm not at all as sure that we cannot have memops in C > that leak into B! > > And B really isn't protected by the lock - it may run while another > CPU still holds the lock, and we know the other CPU released it only > as part of the compare. But that compare isn't a serializing > instruction! > > IOW, I could imagine a load inside C being speculated, and being moved > *ahead* of the load that compares the spinlock head with the tail! > IOW, the load that is _inside_ the spinlock has effectively moved to > outside the protected region, and the spinlock isn't really a reliable > mutual exclusion barrier any more! > > (Yes, there is a data-dependency on the compare, but it is only used > for a conditional branch, and conditional branches are control > dependencies and can be speculated, so CPU speculation can easily > break that apparent dependency chain and do later loads *before* the > spinlock load completes!) > > Now, I have good reason to believe that all Intel and AMD CPU's have a > stricter-than-documented memory ordering, and that your spinlock may > actually work perfectly well. But it still worries me. As far as I can > tell, there's a theoretical problem with your spinlock implementation.
hm, i agree with you that this is problematic. Especially on an SMT CPU it would be a big architectural restriction if prefetches couldnt cross cache misses. (and that's the only way i could see Nick's scheme working: MESI coherency coupled with the speculative use of that cacheline's value never "surviving" a MESI invalidation of that cacheline. That would guarantee that once we have the lock, any speculative result is fully coherent and no other CPU has modified it.) Ingo - 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/