Re: [RFC] [PATCH 1/2] cgroups: read-write lock CLONE_THREAD forking per threadgroup

[Oleg Nesterov <oleg@redhat.com>]

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.