Quoting Benjamin Blum (bblum@xxxxxxxxxx):
> On Mon, Aug 3, 2009 at 1:54 PM, Serge E. Hallyn<serue@xxxxxxxxxx> wrote:
> > Quoting Ben Blum (bblum@xxxxxxxxxx):
> > ...
> >> +static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
> >> + struct task_struct *tsk, int guarantee)
> >> +{
> >> + struct css_set *oldcg;
> >> + struct css_set *newcg;
> >> +
> >> + /*
> >> + * get old css_set. we need to take task_lock and refcount it, because
> >> + * an exiting task can change its css_set to init_css_set and drop its
> >> + * old one without taking cgroup_mutex.
> >> + */
> >> + task_lock(tsk);
> >> + oldcg = tsk->cgroups;
> >> + get_css_set(oldcg);
> >> + task_unlock(tsk);
> >> + /*
> >> + * locate or allocate a new css_set for this task. 'guarantee' tells
> >> + * us whether or not we are sure that a new css_set already exists;
> >> + * in that case, we are not allowed to fail, as we won't need malloc.
> >> + */
> >> + if (guarantee) {
> >> + /*
> >> + * our caller promises us that the css_set we want already
> >> + * exists, so we use find_existing_css_set directly.
> >> + */
> >> + struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
> >> + read_lock(&css_set_lock);
> >> + newcg = find_existing_css_set(oldcg, cgrp, template);
> >> + BUG_ON(!newcg);
> >> + get_css_set(newcg);
> >> + read_unlock(&css_set_lock);
> >> + } else {
> >> + might_sleep();
> >
> > So cgroup_task_migrate() might sleep, but
> >
> > ...
> >
> >
> >> + down_write(&leader->cgroup_fork_mutex);
> >> + rcu_read_lock();
> >> + list_for_each_entry_rcu(tsk, &leader->thread_group, thread_group) {
> >> + /* leave current thread as it is if it's already there */
> >> + oldcgrp = task_cgroup(tsk, subsys_id);
> >> + if (cgrp == oldcgrp)
> >> + continue;
> >> + /* we don't care whether these threads are exiting */
> >> + retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, 1);
> >
> > Here it is called under rcu_read_lock().
>
> You'll notice the fourth argument, which tells cgroup_task_migrate
Hmmm, in my defense one would notice it more readily if the caller used a
meaningful #define instead of '1'.
> whether the css_set is guaranteed or not. If we say we've already got
> it covered, the might_sleep section doesn't happen.
>
> >> -void cgroup_fork(struct task_struct *child)
> >> +void cgroup_fork(struct task_struct *child, int clone_flags)
> >> {
> >> + if (clone_flags & CLONE_THREAD)
> >> + down_read(¤t->group_leader->cgroup_fork_mutex);
> >> + else
> >> + init_rwsem(&child->cgroup_fork_mutex);
> >
> > I'm also worried about the overhead here on what should be a
> > fast case, CLONE_THREAD. Have you done any benchmarking of
> > one thread spawning a bunch of others?
>
> Should be strictly better as this is making the rwsem local to the
> threadgroup - at least in comparison to the previous edition of this
> patch which had it as a global lock.
>
> > What *exactly* is it we are protecting with cgroup_fork_mutex?
> > 'fork' (as the name implies) is not a good answer, since we should be
> > protecting data, not code. If it is solely tsk->cgroups, then perhaps
> > we should in fact try switching to (s?)rcu. Then cgroup_fork() could
> > just do rcu_read_lock, while cgroup_task_migrate() would make the change
> > under a spinlock (to protect against concurrent cgroup_task_migrate()s),
> > and using rcu_assign_pointer to let cgroup_fork() see consistent data
> > either before or after the update... That might mean that any checks done
> > before completing the migrate which involve the # of tasks might become
> > invalidated before the migration completes? Seems acceptable (since
> > it'll be a small overcharge at most and can be quickly remedied).
>
> You'll notice where the rwsem is released - not until cgroup_post_fork
> or cgroup_fork_failed. It doesn't just protect the tsk->cgroups
> pointer, but rather guarantees atomicity between adjusting
> tsk->cgroups and attaching it to the cgroups lists with respect to the
> critical section in attach_proc. If you've a better name for the lock
> for such a race condition, do suggest.
No the name is pretty accurate - it's the lock itself I'm objecting
to. Maybe it's the best we can do, though.
thanks,
-serge
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