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    SubjectRe: [PATCH 10/11] ptrace: move JOBCTL_TRAPPING wait to wait(2) and ptrace_check_attach()
    Hello,

    On Wed, May 11, 2011 at 06:49:47PM +0200, Oleg Nesterov wrote:
    > On 05/08, Tejun Heo wrote:
    > > this patch moves TRAPPING wait from attach completion path to
    > > operations which are actually affected by the transition - wait(2) and
    > > following ptrace(2) requests.
    >
    > You know, I'd wish I could find the serious bugs in this patch. The
    > code becomes really hairy. -EAGAIN in do_wait() doesn't make it more
    > simple ;)

    I don't know. Why is retrying hairy? The whole waiting logic is
    built for clean retries. The suggested change just does it without
    intervening sleeping and waking up. I don't see anything particularly
    hairy there.

    > > Both wait and ptrace paths are updated to retry the operation after
    > > TRAPPING wait. Note that wait_task_stopped() now always grabs siglock
    > > for ptrace waits. This can be avoided with "task_stopped_code() ->
    > > rmb() -> TRAPPING -> rmb() -> task_stopped_code()" sequence
    >
    > And so far I think this would be better, because it seems we can avoid
    > the retry logic.

    Well, the above memory barrier dance wouldn't really change whether
    retry logic is required or not and I'd _really_ like to avoid complex
    barrier dances. Even the typical write-B wmb() write-A / read-A rmb()
    read-B barriers often confuse people. I don't wanna throw in stacked
    wmb()/rmb() pairs there even if that means an extra locking for ptrace
    waits.

    > First of all, this patch returns one of the user-visible and undesirable
    > changes. The tracer know that the task is stopped, attaches, and then it
    > can see the TASK_RUNNING tracee after ptrace(PTRACE_ATTACH) returns.

    Yes, it does. Sorry about forgetting to mention it in the patch
    description. I believe this is something we can swallow.

    > I agree, this looks minor. But if we can tolerate this, probably we can
    > tolerate another oddity: wait_task_stopped() can succeed and eat the
    > stop code before the tracee actually stopps, no?
    >
    > IOW, ignoring mb's and read-ordering, suppose that we simply change
    > task_stopped_code:
    >
    > if (ptrace) {
    > - if (task_is_stopped_or_traced(p))
    > + if (task_is_traced(p) || JOBCTL_TRAPPING)
    > return &p->exit_code;
    > } else {
    >
    > As for ptrace_check_attach(), it can simply do wait_event(), we
    > only need to verify the caller is the tracer. No need to play with
    > lock/unlock/retry.
    >
    > What do you think?

    Hmmm... interesting. Yeah, the state is visible only through wait(2)
    and ptrace(2) and for wait(2) TRAPPING is as good as STOPPED/TRACED
    and we can wait all we want in ptrace_check_attach(). I'll think more
    about it but seems like a nice idea.

    Thank you.

    --
    tejun


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