linux-kernel - Re: BUG: unable to handle kernel paging request from pty

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Message-ID: <CA+55aFyeVmnNuk5pPoH05uPKZRSXt1hv_0PWuvndptqBSfPrbA@mail.gmail.com>
Date:	Thu, 25 Feb 2016 11:09:35 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Jiri Slaby <jslaby@...e.cz>, Greg KH <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stable <stable@...r.kernel.org>, lwn@....net,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: BUG: unable to handle kernel paging request from pty_write [was:
 Linux 4.4.2]

On Thu, Feb 25, 2016 at 10:40 AM, Peter Hurley <peter@...leysoftware.com> 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, 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.

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 ->
            *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. 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