On Thu, Apr 27, 2017 at 12:39 PM, Miroslav Lichvar <mlich...@redhat.com> wrote: > On Thu, Apr 27, 2017 at 12:21:00PM -0400, Willem de Bruijn wrote: >> >> > @@ -720,6 +720,7 @@ void __sock_recv_timestamp(struct msghdr *msg, >> >> > struct sock *sk, >> >> > empty = 0; >> >> > if (shhwtstamps && >> >> > (sk->sk_tsflags & SOF_TIMESTAMPING_RAW_HARDWARE) && >> >> > + (empty || !skb_is_err_queue(skb)) && >> >> > ktime_to_timespec_cond(shhwtstamps->hwtstamp, tss.ts + 2)) { >> >> >> >> I find skb->tstamp == 0 easier to understand than the condition on empty. >> >> >> >> Indeed, this is so non-obvious that I would suggest another helper >> >> function >> >> skb_is_hwtx_tstamp with a concise comment about the race condition >> >> between tx software and hardware timestamps (as in the last sentence of >> >> the commit message). >> > >> > Should it include also the skb_is_err_queue() check? If it returned >> > true for both TX and RX HW timestamps, maybe it could be called >> > skb_has_hw_tstamp? >> >> For the purpose of documenting why this complex condition exists, >> I would call the skb_is_err_queue in that helper function and make >> it tx + hw specific. > > Hm, like this? > > if (shhwtstamps && > (sk->sk_tsflags & SOF_TIMESTAMPING_RAW_HARDWARE) && > + (skb_is_hwtx_tstamp(skb) || !skb_is_err_queue(skb)) && > ktime_to_timespec_cond(shhwtstamps->hwtstamp, tss.ts + 2)) { > > where skb_is_hwtx_tstamp() has > return skb->tstamp == 0 && skb_is_err_queue(skb); > > I was just not sure about the unnecessary skb_is_err_queue() call.
Oh, good point. If the condition is (skb_is_err_queue(skb) && !skb->tstamp) || !skb_is_err_queue(skb) then it makes more sense to just use the simpler expression (!skb_is_err_queue(skb)) || (!skb->tstamp) This cannot be called skb_is_hwtx_tstamp, as this does not imply skb_hwtstamps(skb). Perhaps instead define /* On transmit, software and hardware timestamps are returned independently. * Do not return hardware timestamps even if skb_hwtstamps(skb) is true if * skb->tstamp is set */ static bool skb_is_swtx_tstamp(skb) { return skb_is_err_queue(skb) && skb->tstamp; } and use !skb_is_swtx_tstamp(skb) in this condition. The comment is just a quick first try, can probably be improved.