On 01/03, Ben Blum wrote:
>
> Adds functionality to read/write lock CLONE_THREAD fork()ing per-threadgroup
I didn't actually read this series, but at first glance it still has
problems...
> +struct sighand_struct *threadgroup_fork_lock(struct task_struct *tsk)
static?
> +{
> + struct sighand_struct *sighand;
> + struct task_struct *p;
> +
> + /* tasklist lock protects sighand_struct's disappearance in exit(). */
> + read_lock(&tasklist_lock);
> +
> + /* make sure the threadgroup's state is sane before we proceed */
> + if (unlikely(!thread_group_leader(tsk))) {
> + /* a race with de_thread() stripped us of our leadership */
> + read_unlock(&tasklist_lock);
> + return ERR_PTR(-EAGAIN);
I don't understand how this can close the race with de_thread().
Suppose this tsk is the new leader, after de_thread() changed ->group_leader
and dropped tasklist_lock.
threadgroup_fork_lock() bumps sighand->count
de_thread() continues, notices oldsighand->count != 1 and switches
to the new ->sighand.
After that tsk can spawn other threads, but cgroup_fork() will use
newsighand->threadgroup_fork_lock while cgroup_attach_proc() relies
on oldsighand->threadgroup_fork_lock.
> + /* now try to find a sighand */
> + if (likely(tsk->sighand)) {
> + sighand = tsk->sighand;
> + } else {
> + sighand = ERR_PTR(-ESRCH);
> + /*
> + * tsk is exiting; try to find another thread in the group
> + * whose sighand pointer is still alive.
> + */
> + list_for_each_entry_rcu(p, &tsk->thread_group, thread_group) {
> + if (p->sighand) {
> + sighand = tsk->sighand;
can't understand this "else {}" code... We hold tasklist, if the group
leader is dead (->sighand == NULL), then the whole thread group is
dead.
Even if we had another thread with ->sighand != NULL, what is the point
of "if (unlikely(!thread_group_leader(tsk)))" check then?
Oleg.
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