On Sat, 11 Apr 2009 09:08:54 +0200 Ingo Molnar <mingo@xxxxxxx> 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. > > Ingo The iptables counters are write mostly, read rarely so they don't fit the seq counter or atomic use case. Also, it is important to get a consistent snapshot of the whole set not just each individual counter. -- 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
