Page fault in IFNET_WLOCK_ASSERT [if.c and pccbb.c]

Julian Elischer julian at
Mon Oct 19 04:22:37 UTC 2009

Harsha wrote:
> On Sat, Oct 17, 2009 at 11:42 PM, Julian Elischer <julian at> wrote:
>> Harsha wrote:
>>> <rwatson at> wrote:
>>>> Looks like a NULL pointer dereference, so perhaps a more traditional bug
>>>> --
>>>> could you convert ifindex_alloc_locked+0x71 to a line of code? You can do
>>>> this using kgdb on the kernel symbols file, perhaps "l
>>>> *ifindex_alloc_locked+0x71".
>>> It is the for loop in ifindex_alloc_locked() function-
>>>  for (idx = 1; idx <= V_if_index; idx++)
>>> idx is a local variable, so I figured it is V_if_index is what is
>>> causing the page fault. It does look like a NULL pointer reference - I
>>> see that V_if_index comes from that  vnet instance's value and uses
>>> the macro VNET_VNET_PTR() down the chain. Since the call chain is
>>> coming from a new thread cbb_event_thread, I believe that this
>>> thread's vnet context needs to be set using CURVNET_SET().
>> but only if you have options VIMAGE defined. if not then CURVNET_SET()
>> is a NOP
> I do have the VIMAGE options turned on.
> Can someone tell me what is the right way to add the vnet context to
> cbb_event_thread?

have you read the vimage porting document at:

> I tried adding context in two locations-
> 1. In cbb_insert() as-
> CURVNET_SET(TD_TO_VNET(curthread));
> 2. In if_alloc() as-
> CURVNET_SET(TD_TO_VNET(curthread));
> or as
> ifp = malloc(sizeof(struct ifnet), M_IFNET, M_WAITOK|M_ZERO);
> #ifdef VIMAGE
>       ifp->if_vnet = curvnet;
>       if (ifp->if_home_vnet == NULL)
>           ifp->if_home_vnet = curvnet;
>       CURVNET_SET(ifp->if_vnet);
> #endif

CURVNET_SET sets curvnet but you are using it already, 3 lines above.

> But in all the cases I get a warning/error about unused variable
> 'saved_vnet' like this-
> cc1: warnings being treated as errors
> /usr/src/sys/net/if.c: In function 'if_alloc':
> /usr/src/sys/net/if.c:418: warning: unused variable 'saved_vnet'
> The backtrace is in the link I posted earlier.
> Thanks,
> Harsha

More information about the freebsd-current mailing list