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

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