Hello, Oleg. On Sun, Oct 21, 2012 at 09:11:41PM +0200, Oleg Nesterov wrote: > A couple of off-topic questions. With or without this patch I do not > understand cgroup_fork, > > /* > * We don't need to task_lock() current because current->cgroups > * can't be changed concurrently here. The parent obviously hasn't > * exited and called cgroup_exit(), and we are synchronized against > * cgroup migration through threadgroup_change_begin(). > */ > child->cgroups = current->cgroups; > get_css_set(child->cgroups); > > How so? threadgroup_change_begin() is only called if CLONE_THREAD. > So in theory this copy + atomic_add looks racy... It's a bug. Revert patches to restore task_lock() are already queued in cgroup/for-3.7-fixes. > And it seems that fork() can race with cgroup iterator. post_fork > will notice use_task_css_set_links, but until then the child belongs > to the parent's css and it is not "visible" to iterator (and right > after cgroup_fork() it is not visible to do_each_thread() if > use_task_css_set_links is not set). > > For example. Suppose that the child migrates to another cgroup after > copy_process() makes it visible to the user-space. Then update_if_frozen > sets CGROUP_FROZEN (again, cgroup_iter_next do not see this child). > > Now, post_fork calls freezer_fork() and hits BUG_ON(CGROUP_FROZEN). > > But again, I do not blame this patch. I'm planning to update it to, * Clear ->cgroup to %NULL during copy_process(). A new task isn't considered to be cgroup-wise active at this point. Userland is not allowed to migrate it and none of cgroup callbacks will be called. * Do all the initialization in post_fork so that a task is only allowd to be migrated and operated on cgroup-wise after ->fork() is complete. > I am starting to think again about a big-rw-lock around copy_process. > Recently I tried to add one around dup_mmap for uprobes, but perhaps > cgroups can use it too... If some other subsystems need it, maybe just make threadgroup locking coarser? I *think* I can make cgroup work correctly without a agiant rwlock there but if someone else needs it we can definitely hitch. Thanks. -- tejun _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
