On Thu, 2016-05-19 at 18:50 -0700, Cong Wang wrote: > On Thu, May 19, 2016 at 5:35 AM, Eric Dumazet <eric.duma...@gmail.com> wrote: > > > > These stats are using u64 or u32 fields, so reading integral values > > should not prevent writers from doing concurrent updates if the kernel > > arch is a 64bit one. > > > > Being able to atomically fetch all counters like packets and bytes sent > > at the expense of interfering in fast path (queue and dequeue packets) > > is simply not worth the pain, as the values are generally stale after 1 > > usec. > > I think one purpose of this lock is to make sure we have an atomic > snapshot of these counters as a whole. IOW, we may need another > lock rather than the qdisc root lock to guarantee this.
Right, this was stated in the changelog. I played a bit at changing qdisc->__state to a seqcount. But this would add 2 additional smp_wmb() barriers. Not sure if any application would actually care if it reads Sent 22954160104777 bytes 1808585171 pkt (dropped 0, overlimits 0 requeues 26021) Instead of Sent 22954160104777 bytes 1808585172 pkt (dropped 0, overlimits 0 requeues 26021) ?
