On Mon, Jun 27 2022, Vitaliy Makkoveev <[email protected]> wrote:
> On Sun, Jun 26, 2022 at 10:39:10PM +0200, Alexander Bluhm wrote:
>> On Sun, Jun 26, 2022 at 07:45:57PM +0300, Vitaliy Makkoveev wrote:
>> > This extra serialization is not required. In packet processing path we
>> > have shared netlock held, but we do read-only access on per session
>> > `flags' and `ifindex'. We always modify them from ioctl(2) path with
>> > exclusive netlock held. The rest of pipex(4) session is immutable or
>> > uses per-session locks.
>>
>> I was wondering about pipex_enable variable. It is documented as
>> protected by netlock.
>> net/pipex.c:int pipex_enable = 0; /* [N] */
>>
>> But I did not find NET_LOCK() in pipex_sysctl() or its callers.
>> Should we grab net lock there or in net_sysctl() in the write path?
>> Although only an integer is set atomically, it looks strange if
>> such a value is changed while we are in the middle of packet
>> processing.
>>
>
> I guess it should be declared as atomic. We use ifq_set_maxlen(...,1)
> hack so we don't cross netlock borders in pipex(4) output path, but it's
> expected we could cross them. We never check `pipex_enable' within
> (*if_qstart)() and we don't worry it's not serialized with the rest of
> stack. Also we will process already enqueued pipex(4) packets regardless
> on `pipex_enable' state.
>
> But this means we need to use local copy of `pipex_enable' within
> pppx_if_output(), otherwise we loose consistency.
FWIW pipex_enable_local = READ_ONCE(pipex_enable) would prevent the
compiler from reading the global value twice.
> Index: sys/net/if_pppx.c
> ===================================================================
> RCS file: /cvs/src/sys/net/if_pppx.c,v
> retrieving revision 1.116
> diff -u -p -r1.116 if_pppx.c
> --- sys/net/if_pppx.c 26 Jun 2022 15:50:21 -0000 1.116
> +++ sys/net/if_pppx.c 26 Jun 2022 21:14:02 -0000
> @@ -817,7 +817,9 @@ pppx_if_output(struct ifnet *ifp, struct
> struct pppx_if *pxi = (struct pppx_if *)ifp->if_softc;
> struct pppx_hdr *th;
> int error = 0;
> - int proto;
> + int pipex_enable_local, proto;
> +
> + pipex_enable_local = pipex_enable;
>
> NET_ASSERT_LOCKED();
>
> @@ -831,7 +833,7 @@ pppx_if_output(struct ifnet *ifp, struct
> if (ifp->if_bpf)
> bpf_mtap_af(ifp->if_bpf, dst->sa_family, m, BPF_DIRECTION_OUT);
> #endif
> - if (pipex_enable) {
> + if (pipex_enable_local) {
> switch (dst->sa_family) {
> #ifdef INET6
> case AF_INET6:
> @@ -856,7 +858,7 @@ pppx_if_output(struct ifnet *ifp, struct
> }
> *mtod(m, int *) = proto;
>
> - if (pipex_enable)
> + if (pipex_enable_local)
> error = if_enqueue(ifp, m);
> else {
> M_PREPEND(m, sizeof(struct pppx_hdr), M_DONTWAIT);
> Index: sys/net/pipex.c
> ===================================================================
> RCS file: /cvs/src/sys/net/pipex.c,v
> retrieving revision 1.138
> diff -u -p -r1.138 pipex.c
> --- sys/net/pipex.c 26 Jun 2022 15:50:21 -0000 1.138
> +++ sys/net/pipex.c 26 Jun 2022 21:14:02 -0000
> @@ -94,7 +94,7 @@ struct pool mppe_key_pool;
> * L pipex_list_mtx
> */
>
> -int pipex_enable = 0; /* [N] */
> +int pipex_enable = 0; /* [A] */
> struct pipex_hash_head
> pipex_session_list, /* [L] master session
> list */
> pipex_close_wait_list, /* [L] expired session list */
>
--
jca | PGP : 0x1524E7EE / 5135 92C1 AD36 5293 2BDF DDCC 0DFA 74AE 1524 E7EE
