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Date: Sat, 22 Dec 2012 13:59:11 +0100
From: Dimitry Andric <dim@FreeBSD.org>
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To: Andriy Gapon <avg@FreeBSD.org>
Subject: Re: double fault [Was: clang compiled kernel panic when mounting
 zfs root on i386]
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Cc: freebsd-current@FreeBSD.org, Hans Petter Selasky <hselasky@c2i.net>
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On 2012-12-22 12:11, Andriy Gapon wrote:
> on 21/12/2012 18:38 Hans Petter Selasky said the following:
>> I've built a 10-current i386 kernel as of today, and I see double fault when
>> USB audio is allocating memory. Anyone knows why?
>>
>> kdb_enter()
>> vpanic()
>> panic()
>> dblfault_handler()
>> vm_map_lookup()
>> vm_fault_hold()
>> vm_fault()
>> vm_fault_wire()
>> vm_map_wire()
>> kmem_alloc_attr(xxx, 0x4000,2,0,0xffffffff)
>> bus_dmamem_alloc()
>> usb_pc_alloc_mem()
>
> I suspect that this double fault may have nothing to do with the thread to which
> you followed up.
>
> You need to obtain full debug information to answer your question.

Specifically interesting are the stack frame addresses, which Kostik has
added recently.  From these, you can easily see whether the double fault
is due to stack exhaustion (which seems unlikely with such a small call
stack), or to something else.