Date: Fri, 11 Dec 2015 11:13:58 -0800 From: Randall Stewart <rrs@netflix.com> To: Hans Petter Selasky <hps@selasky.org> Cc: "Alexander V. Chernikov" <melifaro@ipfw.ru>, Adrian Chadd <adrian@freebsd.org>, freebsd-net <freebsd-net@freebsd.org>, Gleb Smirnoff <glebius@FreeBSD.org> Subject: Re: Race between arptimer() and lle removal [WAS: panic in arptimer in r289937] Message-ID: <FC434E75-442B-4E3D-9FFE-936C54823E19@netflix.com> In-Reply-To: <566ABDAF.7060208@selasky.org> References: null <CAJ-VmonvVyTNuYv_as41yPCFdfR5T3FE45DP9MKAc-eyzXzPUg@mail.gmail.com> <2739461446298483@web2h.yandex.ru> <566A94A1.60400@selasky.org> <2850091449828775@web21o.yandex.ru> <566AB081.8050100@selasky.org> <566ABDAF.7060208@selasky.org>
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Hans:
I don=92t think you are getting a 1 back from the callout_reset()..=20
If the pending bit is set, you get a 1 back. But if you have a race =
where
the arp-timer is blocked on the lock (held by arp resolve) your going to
have the pending bit off.. since before calling the function the callout
code removes pending.
Callout-reset() is going to return 0 here since it is running and cannot
be stopped. That should make you *not* decrement your reference.
The only time you get a 1 back (where you will decrement your reference) =
is if
the PENDING flag is set or if you are migrating and the current one =
running. The ARP code
does not migrate that I can see. Which means that once you start =
running
and block a return of 0 will be done by the callout system.
Since you check the PENDING flag at the top of the callout, that would =
get you=20
to return if its been reset.
However I notice down in the arptimer code a unlock/lock/lock is done. =
Now again
the pending flag will be gone when that set of calls is made. So we =
should
be getting a zero return out of the callout_reset() code.
Hmm.
let me mull on this a bit.. any time there is a unlock/lock/lock that =
could be
a problem..
R
On Dec 11, 2015, at 4:12 AM, Hans Petter Selasky <hps@selasky.org> =
wrote:
> On 12/11/15 12:16, Hans Petter Selasky wrote:
>> On 12/11/15 11:12, Alexander V. Chernikov wrote:
>>> 11.12.2015, 12:15, "Hans Petter Selasky" <hps@selasky.org>:
>>>> Hi,
>>>>=20
>>>> Pulling the nail out of the haystack hopefully.
>>>>=20
>>>>>> Any ideas on where next to look?
>>>>=20
>>>> Adrian: In your dump aswell I see:
>>>>=20
>>>> la_flags =3D 1
>>>>=20
>>>> That means there was a race calling arptimer() and removing the =
"lle".
>>> Yes. The interesting part here is why lle is removed. There are =
quite
>>> a few reasons: either interface address deleted or interface going
>>> down, or explicit delete request.
>>> That's why I asked Adrian about interface stuff (and haven't got a
>>> reply).
>>>>=20
>>>> Alexander: Can you comment on the following patch:
>>>>=20
>>>> > Index: netinet/if_ether.c
>>>> > =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>>>> > --- netinet/if_ether.c (revision 291256)
>>>> > +++ netinet/if_ether.c (working copy)
>>>> > @@ -185,7 +185,13 @@
>>>> > LLE_WUNLOCK(lle);
>>>> > return;
>>>> > }
>>>> > - ifp =3D lle->lle_tbl->llt_ifp;
>>>> > + if (lle->la_flags & LLE_LINKED) {
>>>> > + ifp =3D lle->lle_tbl->llt_ifp;
>>>> > + } else {
>>>> > + /* XXX RACE entry has been freed */
>>>> > + llentry_free(lle);
>>>> > + return;
>>>> > + }
>>>> > CURVNET_SET(ifp->if_vnet);
>>>> >
>>>> > if ((lle->la_flags & LLE_DELETED) =3D=3D 0) {
>>>>=20
>>>> We need a check in arptimer() that the lle is still linked before
>>> Yes, I had exactly that approach in mind. (And nd6_llinfo_timer()
>>> needs the same fix).
>>> So, would you commit it or should I?
>>>> proceeding, in there from what I can see. Because the callback is =
not
>>>> protected by a mutex, it is not atomically stopped by =
callout_stop().
>>=20
>> Hi,
>>=20
>> Talking to Randall offlist, I see some more trouble. Let's get
>> everything straight before making a fix. There is one more race I =
see:
>>=20
>> The start of the arptimer() callback looks like this:
>>=20
>> > static void
>> > arptimer(void *arg)
>> > {
>> POINT0
>> > LLE_WLOCK(lle);
>> > if (callout_pending(&lle->lle_timer)) {
>> POINT1
>> > LLE_WUNLOCK(lle);
>> > return;
>> > }
>>=20
>> The code starting the callback looks like this:
>>=20
>> > LLE_ADDREF(la);
>> > la->la_expire =3D time_uptime;
>> > canceled =3D callout_reset(&la->lle_timer, hz *
>> V_arpt_down,
>> > arptimer, la);
>> > if (canceled)
>> > LLE_REMREF(la);
>>=20
>> Which can be written like this:
>>=20
>> > la->la_expire =3D time_uptime;
>> > canceled =3D callout_reset(&la->lle_timer, hz * V_arpt_down,
>> > arptimer, la);
>> > if (canceled =3D=3D 0)
>> > LLE_ADDREF(la);
>>=20
>> In case we are at POINT0 in arptimer, callout_reset() will not be =
able
>> to cancel the callout and will return 0. We should also drop one ref =
at
>> POINT1, or rewrite the code a bit, which might need Randall's help in
>> the callout subsystem area.
>>=20
>=20
> Hi,
>=20
> Looking at the version history, I see Gleb Smirnoff and Randall, =
heavily involved with these code paths. Gleb and Randall, do you have =
any comments on the above findings?
>=20
> Gleb+Randall: Do you agree or disagree there is a race as pointed out =
above?
>=20
> Ways forward:
>=20
> 1) Revert r278472 (done by Randall) and fix r238990 (done by Gleb) to =
use the new callout_async_drain() instead of callout_stop() to avoid =
using the rm lock after free.
>=20
> 2) Use callout_stop() before callout_reset() and add a reference when =
the callout was not pending nor completing. We need to use =
callout_stop() in this case because callout_reset() only has two return =
values and cannot be used to distinguish the three callout states in =
use.
>=20
> 3) Modify callout_reset() to have three return values and fix the =
arptimer code to not leak references.
>=20
> --HPS
--------
Randall Stewart
rrs@netflix.com
803-317-4952
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