Messages in this thread | | | Date | Tue, 5 Jan 2010 19:53:30 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC] [PATCH 1/2] cgroups: read-write lock CLONE_THREAD forking per threadgroup |
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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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