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Date: Mon, 13 Jan 2014 12:19:01 +0100 From: Daniel Borkmann <dborkman@...hat.com> To: Cong Wang <cwang@...pensource.com> CC: David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org> Subject: Re: [PATCH net-next 3/3] packet: use percpu mmap tx frame pending refcount On 01/13/2014 06:51 AM, Cong Wang wrote: > On Sun, Jan 12, 2014 at 8:22 AM, Daniel Borkmann <dborkman@...hat.com> wrote: >> +static void packet_inc_pending(struct packet_ring_buffer *rb) >> +{ >> + this_cpu_inc(*rb->pending_refcnt); >> +} >> + >> +static void packet_dec_pending(struct packet_ring_buffer *rb) >> +{ >> + this_cpu_dec(*rb->pending_refcnt); >> +} >> + >> +static int packet_read_pending(const struct packet_ring_buffer *rb) >> +{ >> + int i, refcnt = 0; >> + >> + /* We don't use pending refcount in rx_ring. */ >> + if (rb->pending_refcnt == NULL) >> + return 0; >> + >> + for_each_possible_cpu(i) >> + refcnt += *per_cpu_ptr(rb->pending_refcnt, i); >> + >> + return refcnt; >> +} > > How is this supposed to work? Since there is no lock, > you can't read accurate refcnt. Take a look at lib/percpu_counter.c. > > I guess for some reason you don't care the accuracy? Yep, not per se. Look at how we do net device reference counting. The reason is that we call packet_read_pending() *only* after we finished processing all frames in TX_RING and we wait for completion in case MSG_DONTWAIT is *not set*, when that happens we're back to 0. But I think I found a different problem with this idea. It could happen with net devices as well, but probably less likely as there might be a better distribution of hold/puts among CPUs. However, for TX_RING, if we pin the process to a particular CPU, and since the destructor is invoked through ksoftirqd, we could end up with a misbalance and if the process runs long enough eventually overflow for one particular CPU. We could work around that, but I think it's not worth the effort. Dave, please drop the 3rd patch of the series, thanks. > Then at least you need to comment in the code. > > Thanks. > -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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