Re: [Xen-devel] [xen] double fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Sun, Oct 6, 2013 at 1:23 AM, Fengguang Wu <fengguang.wu@xxxxxxxxx> wrote:
>
> I got the below dmesg and the first bad commit is commit cf39c8e5352b:
>     Merge tag 'stable/for-linus-3.12-rc0-tag' of 
> git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip

Ugh. How reliable is the double fault? Because bisecting it to the
merge that didn't even have any conflicts in it as far as I can
remember means that there's something really subtle going on wrt some
semantic conflict or other. Or, alternatively, it means that the
bisect failed because the double fault isn't 100% reliable..

Anyway, the stack is crap when the original fault happens at
"boot_tvec_bases+0x1fe", and that causes the double fault debug code
to take *another* fault, which means that it doesn't even show the
right code sequence. Too bad. So ignore the latter part of the oops,
but the top part looks valid:

> [    4.136137] double fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> [    4.137521] CPU: 0 PID: 132 Comm: bootlogd Not tainted 
> 3.12.0-rc2-00153-g14951f2 #129
> [    4.139156] task: ffff88000c9a6580 ti: ffff88000c9ba000 task.ti: 
> ffff88000c9ba000
> [    4.140042] RIP: 0010:[<ffffffff81f31c7e>]  [<ffffffff81f31c7e>] 
> boot_tvec_bases+0x1fe/0x2080
> [    4.140042] RSP: 0018:0000000088000cd8  EFLAGS: 00010212
> [    4.140042] RAX: 000000000000004f RBX: 0000000000000100 RCX: 
> 0000000000000000
> [    4.140042] RDX: 0000000000000f1e RSI: ffffffff81f746a8 RDI: 
> ffffffff81f31c48
> [    4.140042] RBP: ffff88000f003ee0 R08: 0000000000000000 R09: 
> 0000000000000000
> [    4.140042] R10: 0000000000000001 R11: ffff88000f00a000 R12: 
> ffff88000c9bbfd8
> [    4.140042] R13: ffffffff81f31c48 R14: ffffffff81f31c48 R15: 
> ffffffff81f31c48
> [    4.140042] FS:  00007fb1f9662700(0000) GS:ffff88000f000000(0000) 
> knlGS:0000000000000000
> [    4.140042] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    4.140042] CR2: 0000000088000cc8 CR3: 000000000c9cd000 CR4: 
> 00000000000006b0
> [    4.140042] Stack:
<boom, it crashes again here>

but it has jumped into a data section and is executing random data as
code, and there is no sign of where it jumped *from*, since the random
code clearly corrupted the stack - resulting in the double fault in
the first place.

So the oops is almost entirely useless as a debug aid in this
situation. I'm almost hoping that your bisect was wrong, and you could
try to see if you could do that again..

                Linus

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