* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > On Sat, Apr 11, 2009 at 09:08:54AM +0200, Ingo Molnar wrote: > > > > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > I will nevertheless suggest the following egregious hack to > > > get a consistent sample of one counter for some other CPU: > > > > > > a. Disable interrupts > > > b. Atomically exchange the bottom 32 bits of the > > > counter with the value zero. > > > c. Atomically exchange the top 32 bits of the counter > > > with the value zero. > > > d. Concatenate the values obtained in (b) and (c), which > > > is the snapshot value. > > > > Note, i have recently implemented full atomic64_t support on 32-bit > > x86, for the perfcounters code, based on the CMPXCHG8B instruction. > > > > Which, while not the lightest of instructions, is still much better > > than the sequence above. > > > > So i think a better approach would be to also add a dumb generic > > implementation for atomic64_t (using a global lock or so), and then > > generic code could just assume that atomic64_t always exists. > > > > It is far nicer - and faster as well - as the hack above, even on > > 32-bit x86. > > If the generic implementation is needed only on !SMP systems, that > could work. The architectures I would be worried about include > powerpc and ia64, which I believe support 32-bit SMP builds. ia64 would naturally support the CMPXCHG8B instructions. Not sure about powerpc32. Having a lock for the library implementation is not _that_ much of a problem. We obviously dont want the design of Linux to be dictated by the weakest link of all platforms, right? Ingo -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
