Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking

Josh Poimboeuf
2016-04-29T20:11:44Z

On Fri, Apr 29, 2016 at 11:06:53AM -0700, Andy Lutomirski wrote:
> On Thu, Apr 28, 2016 at 1:44 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > A preempted function might not have had a chance to save the frame
> > pointer to the stack yet, which can result in its caller getting skipped
> > on a stack trace.
> >
> > Add a flag to indicate when the task has been preempted so that stack
> > dump code can determine whether the stack trace is reliable.
>
> I think I like this, but how do you handle the rather similar case in
> which a task goes to sleep because it's waiting on IO that happened in
> response to get_user, put_user, copy_from_user, etc?

Hm, good question. I was thinking that page faults had a dedicated
stack, but now looking at the entry and traps code, that doesn't seem to
be the case.

Anyway I think it shouldn't be a problem if we make sure that any kernel
function which might trigger a valid page fault (e.g.,
copy_user_generic_string) do the proper frame pointer setup first. Then
the stack should still be reliable.

In fact I might be able to teach objtool to enforce that: any function
which uses an exception table should create a stack frame.

Or alternatively, maybe set some kind of flag for page faults, similar
to what I did with this patch.

--
Josh