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Subject[PATCH 2/2 v3] cgroup: Drop task_lock(parent) on cgroup_fork()
Date
We don't need to hold the parent task_lock() on the
parent in cgroup_fork() because we are already synchronized
against the two places that may change the parent css_set
concurrently:

- cgroup_exit(), but the parent obviously can't exit concurrently
- cgroup migration: we are synchronized against threadgroup_lock()

So we can safely remove the task_lock() there.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Containers <containers@lists.linux-foundation.org>
Cc: Cgroups <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>
Cc: Mandeep Singh Baines <msb@chromium.org>
---
kernel/cgroup.c | 23 +++++++++++++++++------
1 files changed, 17 insertions(+), 6 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 24f6d6f..4936d88 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4556,20 +4556,31 @@ static const struct file_operations proc_cgroupstats_operations = {
*
* A pointer to the shared css_set was automatically copied in
* fork.c by dup_task_struct(). However, we ignore that copy, since
- * it was not made under the protection of RCU or cgroup_mutex, so
- * might no longer be a valid cgroup pointer. cgroup_attach_task() might
- * have already changed current->cgroups, allowing the previously
- * referenced cgroup group to be removed and freed.
+ * it was not made under the protection of RCU, cgroup_mutex or
+ * threadgroup_change_begin(), so it might no longer be a valid
+ * cgroup pointer. cgroup_attach_task() might have already changed
+ * current->cgroups, allowing the previously referenced cgroup
+ * group to be removed and freed.
+ *
+ * Outside the pointer validity we also need to process the css_set
+ * inheritance between threadgoup_change_begin() and
+ * threadgoup_change_end(), this way there is no leak in any process
+ * wide migration performed by cgroup_attach_proc() that could otherwise
+ * miss a thread because it is too early or too late in the fork stage.
*
* At the point that cgroup_fork() is called, 'current' is the parent
* task, and the passed argument 'child' points to the child task.
*/
void cgroup_fork(struct task_struct *child)
{
- task_lock(current);
+ /*
+ * We don't need to task_lock() current because current->cgroups
+ * can't be changed concurrently here. The parent obviously hasn't
+ * exited and called cgroup_exit(), and we are synchronized against
+ * cgroup migration through threadgroup_change_begin().
+ */
child->cgroups = current->cgroups;
get_css_set(child->cgroups);
- task_unlock(current);
INIT_LIST_HEAD(&child->cg_list);
}

--
1.7.5.4


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