Deterministic panic due to non-sleepable lock with if_alc when reconfiguring interfaces

Garrett Cooper yanegomi at
Fri Aug 19 07:17:13 UTC 2011

On Thu, Aug 18, 2011 at 9:31 PM,  <mdf at> wrote:
> On Thu, Aug 18, 2011 at 5:50 PM, Garrett Cooper <yanegomi at> wrote:
>>    When loading if_alc as a module on my netbook and running
>> /etc/rc.d/netif restart, I can deterministically panic my netbook with
>> the following message:

    These repro steps were overly simplified. The complete steps are:

1. Attach ethernet cable to alc(4) enabled NIC.
2. Boot up machine.
3. Login.
4. Physically remove ethernet cable from alc(4) enabled NIC.
5. Run `/etc/rc.d/netif restart' as root.

>> ) at _bus_dmamap_sync+0x51
>> alc_stop(c3dbb000,0,c0c51844,93a,80206910,...) at alc_stop+0x24e
>> alc_ioctl(c3d07400,80206910,c40423c0,c06a7935,c0914e3c,...) at alc_ioctl+0x22e
>> ifioctl(c45029c0,80206910,c40423c0,c40505c0,c4528c00,...) at ifioctl+0xc98
>> soo_ioctl(c4574e00,80206910,c40423c0,c413e680,c40505c0,...) at soo_ioctl+0x401
>> kern_ioctl(c40505c0,3,80206910,c40423c0,c40423c0,...) at kern_ioctl+0x1d7
>> ioctl(c40505c0,e6ca3cec,e6ca3d28,c08e929d,0,...) at ioctl+0x118
>> syscallenter(c40505c0,e6ca3ce4,e6ca3ce4,0,0,...) at syscallenter+0x23f
>> syscall(e6ca3d28) at syscall+0x2e
>> Xint0x80_syscall() at Xint0x80_syscall+0x21
>> --- syscall (54kernel trap 12 with interrupts disabled
>> Kernel page fault with the following non-sleepable locks held:
>> exclusive sleep mutex alc0 (network driver) r = 0 (0xc3dbc608) locked
>> @ /usr/src/sys/modules/alc/../../dev/alc/if_alc.c:2362
>> KDB: stack backtrace:
>> db_trace_self_wrapper(c08e727a,80,6e726500,74206c65,20706172,...) at
>> db_trace_self_wrapper+0x26
>> kdb_backtrace(93a,0,ffffffff,c0ad6114,e6ca323c,...) at kdb_backtrace+0x2a
>> _witness_debugger(c08e9f67,e6ca3250,4,1,0,...) at _witness_debugger+0x1e
>> witness_warn(5,0,c0924fe1,c097df50,c3e42b00,...) at witness_warn+0x1f1
>> trap(e6ca32dc) at trap+0x15a
>> calltrap() at calltrap+0x6
>>    I tried to track down what the exact issue was, but I got lost
>> (the locking sort of looks ok to me, but I'm still not an expert with
>> mutex(9)).
>>    I still have the vmcore and can provide more helpful details when requested.
> The locking itself is almost certainly fine.  The error message is not
> very helpful, but what went wrong was the page fault.  You just happen
> to panic on a witness warning before vm_fault can panic due to a bad
> address.
> The alc(4) maintainer would probably like info on the trap (line of
> code and where the bad pointer came from).

    I talked to Xin a bit and as he noted the panic was just a symptom
of the actual issue at hand. I think the problem is that the rx ring's
rx_m value isn't set to NULL when an error occurred, but getting to
the exact problem at hand, the following call is failing:

        if (bus_dmamap_load_mbuf_sg(sc->alc_cdata.alc_rx_tag, // <-- HERE
            sc->alc_cdata.alc_rx_sparemap, m, segs, &nsegs, 0) != 0) {
                return (ENOBUFS);

    It's failing with ENOMEM. Still trying to determine what the exact
reason for ENOMEM is from the x86 busdma code though..

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