On Sun, Jun 26, 2022 at 11:26:13PM +0200, Jeremie Courreges-Anglas wrote:
> 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.
Or use atomic_load_int() to prevent over optimization.
I would prefer atomic_...() access for [A] variables.
bluhm
> > 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
