On Fri, Dec 23, 2011 at 10:40:28AM +0800, Li Zefan wrote:
> >> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> >> index 1042b3c..032139d 100644
> >> --- a/kernel/cgroup.c
> >> +++ b/kernel/cgroup.c
> >> @@ -2102,21 +2102,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
> >> if (retval)
> >> goto out_free_group_list;
> >>
> >> - /* prevent changes to the threadgroup list while we take a snapshot. */
> >> - read_lock(&tasklist_lock);
> >> - if (!thread_group_leader(leader)) {
> >> - /*
> >> - * a race with de_thread from another thread's exec() may strip
> >> - * us of our leadership, making while_each_thread unsafe to use
> >> - * on this task. if this happens, there is no choice but to
> >> - * throw this task away and try again (from cgroup_procs_write);
> >> - * this is "double-double-toil-and-trouble-check locking".
> >> - */
> >> - read_unlock(&tasklist_lock);
> >> - retval = -EAGAIN;
> >> - goto out_free_group_list;
> >> - }
> >> -
> >> tsk = leader;
> >> i = 0;
> >> do {
> >> @@ -2145,7 +2130,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
> >> group_size = i;
> >> tset.tc_array = group;
> >> tset.tc_array_len = group_size;
> >> - read_unlock(&tasklist_lock);
> >
> > You still need rcu_read_lock()/rcu_read_unlock() around
> > do {
> >
> > } while_each_thread()
> >
> > because threadgroup_lock() doesn't lock the part that remove a thread from
> > its group on exit.
> >
>
> and inside rcu critical section, you can't call kmalloc(GFP_KERNEL)!!
Good point. Well it's still worth replacing tasklist_lock by rcu though :)
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