Re: rtentry_free panic

On 20 Aug 2025, at 18:00, Mark Johnston wrote:
> On Wed, Aug 20, 2025 at 02:30:20PM +0200, Kristof Provost wrote:
>> We’re panicing because the V_rtzone zone has been cleaned up (in
>> vnet_rtzone_destroy()). I explicitly NULL out V_rtzone too, to make 
>> this
>> more obvious.
>> Note that we failed to completely free all rtentries (`Freed UMA keg
>> (rtentry) was not empty (2 items).  Lost 1 pages of memory.`). 
>> Presumably at
>> least on of those two gets freed later, and that’s the panic we 
>> see.
>>
>> rt_free() queues the actual delete as an epoch callback
>> (`NET_EPOCH_CALL(destroy_rtentry_epoch, &rt->rt_epoch_ctx);`), and 
>> that’s
>> what we see here: the zone is removed before we’re done freeing all 
>> of the
>> rtentries.
>>
>> vnet_rtzone_destroy() is called from rtables_destroy(), but that 
>> explicitly
>> calls NET_EPOCH_DRAIN_CALLBACKS() first, so I’d expect all of the 
>> pending
>> cleanups to have been done at that point.  The comment block above 
>> does
>> suggest that there may still be nexthop entries pending deletion even 
>> after
>> the we drain the callbacks. I think I can see how that’d happen for
>> nexthops, but I do not see how it can happen for rtentries.
>
> Is it possible that if_detach_internal()->rt_flushifroutes() is 
> running
> after the rtentry zone is being destroyed?  That is, maybe we're
> destroying interfaces too late in the jail teardown process?
>
With a little work to pass the calling function and line number through 
the call stack (and a lot of patience to reproduce the panic) I think 
I’ve found where we initially rt_free() the relevant rtentry, but 
it’s left me even more confused.

The call happens from ip6_destroy() -> in6_purgeaddr() -> 
ifa_del_loopback_route() -> ifa_maintain_loopback_route() -> 
rib_action() -> rib_del_route() -> rt_free().
That’s a NET_EPOCH_CALL(), which should be fine because in 
rtables_destroy() we NET_EPOCH_CALLBACK_DRAIN() before we 
vnet_rtzone_destory() (which naturally destroys the relevant uma zone).

ip6_destroy()’s VNET_SYSUNIT is SI_SUB_PROTO_DOMAIN/SI_ORDER_THIRD and 
rtables_destroy()’s is SI_SUB_PROTO_DOMAIN/SI_ORDER_FIRST. Given that 
it’s *un*init that means we call ip6_destroy() first, so that should 
all just work.
The enqueued freeing of the rtentries should all be handled once 
NET_EPOCH_CALLBACK_DRAIN completes, but that appears to not be the case.

—
Kristof