Thanks a lot Magnus Now I fully understand. Julius On Mon, Jul 10, 2023 at 3:57 PM Magnus Karlsson <magnus.karlsson@xxxxxxxxx> wrote: > > On Mon, 10 Jul 2023 at 15:51, Július Milan <julius.milan.22@xxxxxxxxx> wrote: > > > > Hi all > > > > I am missing a full understanding of what really is thread safe and > > what is not, when using AF_XDP. > > > > RX and TX operations look obvious as every xsk socket has its own RX > > and TX rings and so I can have multiple threads, as long as they don't > > try to use the same ring of the same socket concurrently, that's the > > only limitation (Am I correct yet?). So I can have for example 1:1 > > mapping of thread to xsk socket to avoid locking. > > That is correct. > > > However what about UMEM rings, i.e. Completion and Fill queue? > > About ring operations as __peek, __reserve ..., it is written (for ex. > > here: https://www.kernel.org/doc/html/latest/networking/af_xdp.html#rings), > > that they are not thread safe. > > So it seems that I should use mutex to access them. > > But I can see in: > > https://github.com/xdp-project/bpf-examples/blob/master/AF_XDP-forwarding/xsk_fwd.c#L526 > > That they are filled / drained without locking, how is it then? > > The documentation above mentions that there are two shared_umem modes: > one if you want to share the same netdev/queue_id and another one for > all the other cases. The forwarding example uses the latter which has > one fill ring and one completion ring per socket. So no locking > needed. The former only has one fill and completion ring pair per > umem, so you need to take care of mutual exclusion yourself in this > case. > > > Does it make a difference if the socket is created by > > xsk_socket__create_shared or xsk_socket__create? > > The former one sets the XDP_SHARED_UMEM flag. If you want to share the > umem between several sockets, you need to use this one. > > > What is the limitation that cannot be broken, as I described it for RX > > / TX? (if correctly) > > > > Best Regards > > Julius
|