cgroup: replace tasklist_lock with rcu_read_lock

We can replace the tasklist_lock in cgroup_attach_proc with an
rcu_read_lock().

Changes in V4:
* https://lkml.org/lkml/2011/12/23/284 (Frederic Weisbecker)
  * Minimize size of rcu_read_lock critical section
  * Add comment
* https://lkml.org/lkml/2011/12/26/136 (Li Zefan)
  * Split into two patches
Changes in V3:
* https://lkml.org/lkml/2011/12/22/419 (Frederic Weisbecker)
  * Add an rcu_read_lock to protect against exit
Changes in V2:
* https://lkml.org/lkml/2011/12/22/86 (Tejun Heo)
  * Use a goto instead of returning -EAGAIN

Suggested-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: containers@lists.linux-foundation.org
Cc: cgroups@vger.kernel.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 12c07e8fd69c..1626152dcc1e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2102,10 +2102,14 @@ static 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);
         tsk = leader;
         i = 0;
+        /*
+         * Prevent freeing of tasks while we take a snapshot. Tasks that are
+         * already PF_EXITING could be freed from underneath us unless we
+         * take an rcu_read_lock.
+         */
+        rcu_read_lock();
         do {
                 struct task_and_cgroup ent;
 
@@ -2128,11 +2132,11 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
                 BUG_ON(retval != 0);
                 i++;
         } while_each_thread(leader, tsk);
+        rcu_read_unlock();
         /* remember the number of threads in the array for later. */
         group_size = i;
         tset.tc_array = group;
         tset.tc_array_len = group_size;
-        read_unlock(&tasklist_lock);
 
         /* methods shouldn't be called if no task is actually migrating */
         retval = 0;