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SubjectRe: doublefault debugging (was Re: Linux v2.5.62 --- spontaneous reboots)

On Thu, 20 Feb 2003, Ingo Molnar wrote:
> i think i managed to trigger a potentially useful oops, with BK-curr:

Ok, this is definitely a stack overflow:

> EIP is at do_page_fault+0x7b/0x4e4
> eax: 6b6b6b8b ebx: 6b6b6b6b ecx: 0000002b edx: c02dd6ac
> esi: 6b6b6b8b edi: ca095320 ebp: ca092170 esp: ca0920c8
> ds: 007b es: 007b ss: 0068
> Process start-threads (pid: 21685, threadinfo=ca090000 task=ca094ce0)

Note the "threadinfo=ca090000" and "esp: ca0920c8".

If the threadinfo isn't on the same double-page as the stack, then you're
screwed, and you've just overwritten the _real_ threadinfo, and the stack
is probably screwed. In fact, any recursion on do_page_fault() is
_probably_ due to the fact that you overwrote thread-info.

This could explain Chris' problems too - my doublefault thing won't help
much if recursion on the stack has clobbered a lot of kernel state (and
the doublefault will likely happen only after enough state is clobbered
that even the doublefault handling might have trouble).

> [tons of pagefault recursion]
> [<c01193d0>] do_page_fault+0x0/0x4e4
> [<c010a691>] error_code+0x2d/0x38
> [<c011944b>] do_page_fault+0x7b/0x4e4
> [<c01193d0>] do_page_fault+0x0/0x4e4
> [<c010a691>] error_code+0x2d/0x38
> [<c011944b>] do_page_fault+0x7b/0x4e4
> [<c01294f8>] do_timer+0xc8/0xd0
> [<c013330c>] rcu_process_callbacks+0x17c/0x1b0
> [<c011b4bf>] scheduler_tick+0x3ff/0x410
> [<c0125113>] tasklet_action+0x73/0xc0
> [<c01193d0>] do_page_fault+0x0/0x4e4
> [<c010a691>] error_code+0x2d/0x38
> [<c011b598>] schedule+0xb8/0x3d0
> [<c01219fd>] release_task+0x17d/0x200
> [<c011e70f>] mmput+0x1f/0xc0
> [<c0122cad>] do_exit+0x31d/0x3b0
> [<c010b328>] do_nmi+0x58/0x60
> [<c012a93e>] __dequeue_signal+0x6e/0xb0
> [<c0122ef0>] do_group_exit+0x110/0x140
> [<c012a9ae>] dequeue_signal+0x2e/0x60
> [<c012c2b1>] get_signal_to_deliver+0x2b1/0x440
> [<c01099a2>] do_signal+0xb2/0xf0
> [<c01296c4>] schedule_timeout+0x74/0xc0
> [<c012c4f9>] sigprocmask+0x89/0x140
> [<c0129640>] process_timeout+0x0/0x10
> [<c012c62d>] sys_rt_sigprocmask+0x7d/0x1a0
> [<c0129944>] sys_nanosleep+0x154/0x180
> [<c0109a3b>] do_notify_resume+0x5b/0x60
> [<c0109c72>] work_notifysig+0x13/0x15

I bet the doublefaults are on "tsk->mm" accesses (specifically,
tsk->mm->mmap_sem", which should be the first of them).

That easily happens if "tsk" is crud (either because recursion has already
overwritten it, _or_ because %esp has recursed so far down that the
"current()" logic ends up hitting the next page.

The stack doesn't look _that_ deep to me, but if some of these functions
have a large local frame, then that would certainly do it.. At a guess, it
Looks like a fairly deep "schedule()" coupled with deep RCU processing.

And that RCU path is reasonably new. The infrastructure was put in 2.5.43,
which might explain Chris' case too ("somewhere before 2.5.51").

Does anybody have an up-to-date "use -gp and a special 'mcount()'
function to check stack depth" patch? The CONFIG_DEBUG_STACKOVERFLOW thing
is quite possibly too stupid to find things like this (it only finds
interrupts that overflow the stack, not deep call sequences).

Guys: you could try to enable CONFIG_DEBUG_STACKOVERFLOW, and then perhaps
make it a bit more aggressive (rigth now it does:

if (unlikely(esp < (sizeof(struct thread_info) + 1024))) {

and I'd suggest changing it to something more like

/* Have we used up more than half the stack? */
if (unlikely(esp < 4096)) {

and add a "for (;;)" after doing the dump_stack() because otherwise the
machine may reboot before you get anywhere.


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