Re: [PATCH][BUGFIX] cgroups: more safe tasklist locking incgroup_attach_proc

From: Oleg Nesterov
Date: Mon Aug 15 2011 - 14:54:07 EST

On 07/29, Ben Blum wrote:
> According to this thread - - RCU is
> not sufficient to guarantee the tasklist is stable w.r.t. de_thread and
> exit. Taking tasklist_lock for reading, instead of rcu_read_lock,
> ensures proper exclusion.


So far I still think we should fix while_each_thread() so that it works
under rcu_read_lock() "as exepected", I'll try to think more.

But whatever we do with while_each_thread(), this can't help
cgroup_attach_proc(), it needs the locking.

> - rcu_read_lock();
> + read_lock(&tasklist_lock);
> if (!thread_group_leader(leader)) {

Agreed, this should work.

But can't we avoid the global list? thread_group_leader() or not, we do
not really care. We only need to ensure we can safely find all threads.

How about the patch below?

With or without this/your patch this leader can die right after we
drop the lock. ss->can_attach(leader) and ss->attach(leader) look
suspicious. If a sub-thread execs, this task_struct has nothing to
do with the threadgroup.

Also. This is off-topic, but... Why cgroup_attach_proc() and
cgroup_attach_task() do ->attach_task() + cgroup_task_migrate()
in the different order? cgroup_attach_proc() looks wrong even
if currently doesn't matter.


--- x/kernel/cgroup.c
+++ x/kernel/cgroup.c
@@ -2000,6 +2000,7 @@ int cgroup_attach_proc(struct cgroup *cg
/* threadgroup list cursor and array */
struct task_struct *tsk;
struct flex_array *group;
+ unsigned long flags;
* we need to make sure we have css_sets for all the tasks we're
* going to move -before- we actually start moving them, so that in
@@ -2027,19 +2028,10 @@ int cgroup_attach_proc(struct cgroup *cg
goto out_free_group_list;

/* prevent changes to the threadgroup list while we take a snapshot. */
- rcu_read_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".
- */
- rcu_read_unlock();
- retval = -EAGAIN;
+ retval = -EAGAIN;
+ if (!lock_task_sighand(leader, &flags))
goto out_free_group_list;
- }
/* take a reference on each task in the group to go in the array. */
tsk = leader;
i = 0;
@@ -2055,9 +2047,9 @@ int cgroup_attach_proc(struct cgroup *cg
BUG_ON(retval != 0);
} while_each_thread(leader, tsk);
+ unlock_task_sighand(leader, &flags);
/* remember the number of threads in the array for later. */
group_size = i;
- rcu_read_unlock();

* step 1: check that we can legitimately attach to the cgroup.

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