Re: BUG: unable to handle kernel paging request from pty_write [was: Linux 4.4.2]

[Peter Hurley]

On 02/25/2016 11:09 AM, Linus Torvalds wrote:
> On Thu, Feb 25, 2016 at 10:40 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
>>
>> The crash itself is in try_to_wake_up() (again, assuming the stacktrace is
>> valid).
> 
> No, the crash seems to be off in la-la-land

I meant the last-known-good address is try_to_wake_up(); in the same way
that RIP @ 0 crashes, but no one says the crash is @ NULL.


>, judging by the oops:
> 
>    IP: [<ffff88023fd40000>] 0xffff88023fd40000
> 
> which isn't kernel code at all. It is close to, but not at, the percpu
> area you point out.

Assuming ffff88023fdc0000 is percpu start for cpu 7 then I'm pretty sure
         ffff88023fd40000 is percpu start for cpu 6.

Either way, RIP is almost certainly in the percpu block.


> But yes, the call trace looks accurate and makes sense, we haveL
> 
>   tty_flip_buffer_push ->
>     (queue_work is inline) ->
>     queue_work_on ->
>       __queue_work ->
>         insert_work ->
>           (wake_up_worker is inlined)
>           wake_up_process ->

              try_to_wake_up ->

>             *insane non-code address*
> 
> but I cannot for the life of me see how we get to an insane address.
> It smells like stack corruption when returning from try_to_wake_up()
> or something like that.
> 
> Hmm. Actually, try_to_wake_up() will do several indirect calls
> (task_waking and select_task_rq, and it_func_ptr->fn for tracing), but
> then I'd expect to see try_to_wake_up itself in the stack trace.


> Of course, when you jump to la-la-land, crazy things can happen. And
> that offending IP is at a page boundary, so it migth have run some
> random code on the previous page.
> 
> Quite frankly, neither ->task_waking() nor ->select_task_rq() look
> very likely.

Agreed, the sched_class indirections do not seem likely.


> But the tracepoint stuff is actually fairly dynamic, and
> does things like
> 
>     it_func_ptr = rcu_dereference_sched((tp)->funcs);
> 
> to get the function pointer information, so if there is some race in
> there, anything can happen.
> 
> Jiri, were you messing around with tracing when this happened? Or
> maybe shutting down CPU's? There was a RCU locking problem with CPU
> shutdown, maybe this is one of the symptoms. The fix for that is
> recent, and not in 4.4.2.
> 
> Adding Steven Rostedt to the cc. Steven, does that look like a possible case?
> 
>                         Linus
>