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    SubjectRe: while_each_thread() under rcu_read_lock() is broken?
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    On Fri, Jun 18, 2010 at 12:34 PM, Oleg Nesterov <oleg@redhat.com> wrote:
    > (add cc's)
    >
    > Hmm. Once I sent this patch, I suddenly realized with horror that
    > while_each_thread() is NOT safe under rcu_read_lock(). Both
    > do_each_thread/while_each_thread or do/while_each_thread() can
    > race with exec().
    >
    > Yes, it is safe to do next_thread() or next_task(). But:
    >
    >        #define while_each_thread(g, t) \
    >                while ((t = next_thread(t)) != g)
    >
    > suppose that t is not the group leader, and it does de_thread() and then
    > release_task(g). After that next_thread(t) returns t, not g, and the loop
    > will never stop.
    >
    > I _really_ hope I missed something, will recheck tomorrow with the fresh
    > head. Still I'd like to share my concerns...
    >

    Yep. You're right. Not sure what I was thinking. This is only case
    where do_each_thread
    is protected by an rcu_read_lock. All others, correctly use read_lock.

    > If I am right, probably we can fix this, something like
    >
    >        #define while_each_thread(g, t) \
    >                while ((t = next_thread(t)) != g && pid_alive(g))
    >

    This seems like a reasonable approach. Maybe call it:

    while_each_thread_maybe_rcu() :)

    This makes hung_task a little less useful for failure fencing since
    this (and rcu_lock_break)
    increases the potential for never examining all threads but its still
    a nice lightweight diagnostic
    for finding bugs.

    > [we can't do while (!thread_group_leadr(t = next_thread(t)))].
    > but this needs barrires, and we should validate the callers anyway.
    >
    > Or, perhaps,
    >
    >        #define XXX(t)  ({
    >                struct task_struct *__prev = t;
    >                t = next_thread(t);
    >                t != g && t != __prev;
    >        })
    >
    >        #define while_each_thread(g, t) \
    >                while (XXX(t))
    >
    > Please tell me I am wrong!
    >
    > Oleg.
    >
    > On 06/18, Oleg Nesterov wrote:
    >>
    >> check_hung_uninterruptible_tasks()->rcu_lock_break() introduced by
    >> "softlockup: check all tasks in hung_task" commit ce9dbe24 looks
    >> absolutely wrong.
    >>
    >>       - rcu_lock_break() does put_task_struct(). If the task has exited
    >>         it is not safe to even read its ->state, nothing protects this
    >>         task_struct.
    >>
    >>       - The TASK_DEAD checks are wrong too. Contrary to the comment, we
    >>         can't use it to check if the task was unhashed. It can be unhashed
    >>         without TASK_DEAD, or it can be valid with TASK_DEAD.
    >>
    >>         For example, an autoreaping task can do release_task(current)
    >>         long before it sets TASK_DEAD in do_exit().
    >>
    >>         Or, a zombie task can have ->state == TASK_DEAD but release_task()
    >>         was not called, and in this case we must not break the loop.
    >>
    >> Change this code to check pid_alive() instead, and do this before we
    >> drop the reference to the task_struct.
    >>
    >> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    >> ---
    >>
    >>  kernel/hung_task.c |   11 +++++++----
    >>  1 file changed, 7 insertions(+), 4 deletions(-)
    >>
    >> --- 35-rc2/kernel/hung_task.c~CHT_FIX_RCU_LOCK_BREAK  2009-12-18 19:05:38.000000000 +0100
    >> +++ 35-rc2/kernel/hung_task.c 2010-06-18 20:06:11.000000000 +0200
    >> @@ -113,15 +113,20 @@ static void check_hung_task(struct task_
    >>   * For preemptible RCU it is sufficient to call rcu_read_unlock in order
    >>   * exit the grace period. For classic RCU, a reschedule is required.
    >>   */
    >> -static void rcu_lock_break(struct task_struct *g, struct task_struct *t)
    >> +static bool rcu_lock_break(struct task_struct *g, struct task_struct *t)
    >>  {
    >> +     bool can_cont;
    >> +
    >>       get_task_struct(g);
    >>       get_task_struct(t);
    >>       rcu_read_unlock();
    >>       cond_resched();
    >>       rcu_read_lock();
    >> +     can_cont = pid_alive(g) && pid_alive(t);
    >>       put_task_struct(t);
    >>       put_task_struct(g);
    >> +
    >> +     return can_cont;
    >>  }
    >>
    >>  /*
    >> @@ -148,9 +153,7 @@ static void check_hung_uninterruptible_t
    >>                       goto unlock;
    >>               if (!--batch_count) {
    >>                       batch_count = HUNG_TASK_BATCHING;
    >> -                     rcu_lock_break(g, t);
    >> -                     /* Exit if t or g was unhashed during refresh. */
    >> -                     if (t->state == TASK_DEAD || g->state == TASK_DEAD)
    >> +                     if (!rcu_lock_break(g, t))
    >>                               goto unlock;
    >>               }
    >>               /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
    >
    >
    --
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