Re: [PATCH 1/7] cgroup: cgroup_subsys->fork() should be called after the task is added to css_set

ÓÚ 2012/10/17 16:28, Li Zefan дµÀ:

On 2012/10/17 6:28, Tejun Heo wrote:

quoted

cgroup core has a bug which violates a basic rule about event
notifications - when a new entity needs to be added, you add that to
the notification list first and then make the new entity conform to
the current state.  If done in the reverse order, an event happening
inbetween will be lost.

cgroup_subsys->fork() is invoked way before the new task is added to
the css_set.  Currently, cgroup_freezer is the only user of ->fork()
and uses it to make new tasks conform to the current state of the
freezer.  If FROZEN state is requested while fork is in progress
between cgroup_fork_callbacks() and cgroup_post_fork(), the child
could escape freezing - the cgroup isn't frozen when ->fork() is
called and the freezer couldn't see the new task on the css_set.

This patch moves cgroup_subsys->fork() invocation to
cgroup_post_fork() after the new task is added to the css_set.
cgroup_fork_callbacks() is removed.

Because now a task may be migrated during cgroup_subsys->fork(),
freezer_fork() is updated so that it adheres to the usual RCU locking
and the rather pointless comment on why locking can be different there
is removed (if it doesn't make anything simpler, why even bother?).

I don't think rcu read section is sufficient. It guarantees the data you're
accessing is valid, but the data can be new or can be old.

So a case below is possible:

in freezer_fork():
rcu_read_lock();
freezer = task_freezer(task);
                                  move task from freezer to freezer2
                                  which is in FREEZING/FROZEN state
freezer is in THAWED state,
nothing to do.
rcu_read_unlock();

forget about it. The task will be correctly frozen when moving to
another cgroup, so nothing unexpected will happen.

for this patch:

Acked-by: Li Zefan <redacted>