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    SubjectRe: Q: cgroup: Questions about possible issues in cgroup locking
    Hi Oleg,

    Oleg Nesterov (oleg@redhat.com) wrote:
    > Hi Mandeep,
    >
    > On 01/11, Mandeep Singh Baines wrote:
    > > > >
    > > > > #define while_each_thread(g, t, o) \
    > > > > while (t->group_leader == o && (t = next_thread(t)) != g)
    > > > >
    > > > > Where o should have the value of g->group_leader.
    > > >
    > > > I don't understand how this helps... and how this can work even
    > > > ignoring the barriers.
    > > >
    > > > OK, we have the main thream M and the sub-thread T, we are doing
    > > >
    > > > do {
    > > > do_something(t);
    > > > } while_each_thread(M, t, M);
    > > >
    > > > why we can't miss T if it does exec?
    > > >
    > >
    > > So for:
    > >
    > > struct task *M; /* assuming this is passed in to us */
    > > struct task *L = M->group_leader;
    >
    > L == M
    >
    > > do {
    > > do_something(T);
    > > } while_each_thread(M, T, L);
    > >
    > > Here is my thinking.
    > >
    > > If some thread K does exec, you won't miss it because:
    > >
    > > 1) Ignoring the group_leader check, you'll visit K just by following
    > > next_thread(). That's the case today and is what you except
    > > when iterating over an rcu_list.
    > > 2) (t->group_leader == o) will fail iff t is the exec thread.
    > > Since we test t->group_leader before re-assigning it (t=next_thread()),
    > > the test will fail only after visiting the exec thread. So you'll
    > > visit the exec thread and then terminate the loop.
    >
    > Still can't understand... Lets look at this trivial example again.
    >
    > We start from the main thread M, it is ->group_leader. There is
    > another thread T in this thread group. We are doing
    >
    > OLD = M;
    >
    > t = M;
    > do {
    > do_smth(t);
    > }
    > while (t->group_leader == OLD && ((t = next_thread(t)) != M);
    >
    > The first iteration does do_smth(M).
    >
    > T calls de_thread() and, in particular, it does M->group_leader = T
    > (see "leader->group_leader = tsk" in de_thread).
    >
    > after that t->group_leader == OLD fails. t == M, its group_leader == T.
    > do_smth(T) won't be called.
    >
    > No?
    >

    I think we can handle this by removing the assignment. So in de_thread():

    - leader->group_leader = tsk;

    tsk->exit_signal = SIGCHLD;
    leader->exit_signal = -1;

    BUG_ON(leader->exit_state != EXIT_ZOMBIE);
    leader->exit_state = EXIT_DEAD;

    In the current d_thread(), four statements after reassigning
    leader->group_leader, we mark the old leader as EXIT_DEAD. So what if
    we leave leader->group_leader = leader. Since its EXIT_DEAD a few
    statements later, I don't think anything should break.

    What do you think?

    Regards,
    Mandeep

    > Oleg.
    >


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