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    SubjectRe: [PATCH]: Re: kernel 2.6.9-rc1-mm4 oops

    * Kirill Korotaev <dev@sw.ru> wrote:

    > This patch removes sighand checks from the next_thread(), since they
    > are incorrect and has nothing to do with the next_thread() function.
    > So they could trigger BUG() when there were no actually bug at all.

    the problem is, generally it is not valid to have a thread on the thread
    list that has no ->sighand structure. This is what happens when we exit
    a task:

    write_lock_irq(&tasklist_lock);
    ...
    __exit_sighand(p);
    __unhash_process(p);

    the BUG() is useful for all the code that uses next_thread() - you can
    only do a safe next_thread() iteration if you've locked ->sighand.

    there's one exception: in the procfs code we can get a reference to
    almost-dead tasks as well that are not even in the tasklist. (This is a
    relatively new thing introduced by me that can happen due to the
    preemptability of some of the exit path.)

    so i believe your fix papers over the real bug which is the use of an
    almost-dead task for thread iterations. Since we've already done
    __unhash_process() not doing the BUG introduces a more subtle bug: the
    use of the stale PID pointers! So i believe the right fix is the one
    below, which (under the safety of read_lock(tasklock)) checks for the
    availability of task->sighand - and skips the thread iterations if so.

    Ingo

    --- linux/fs/proc/array.c.orig
    +++ linux/fs/proc/array.c
    @@ -356,7 +356,7 @@ static int do_task_stat(struct task_stru
    stime = task->signal->stime;
    }
    }
    - if (whole) {
    + if (whole && task->sighand) {
    t = task;
    do {
    min_flt += t->min_flt;
    -
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