The commit is pushed to "branch-rh7-3.10.0-1160.36.2.vz7.182.x-ovz" and will
appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1160.36.2.vz7.182.1
------>
commit d5e8cbbf52cbfd6e94df2c819324c6c3f81d5c97
Author: Pavel Tikhomirov <[email protected]>
Date: Tue Aug 24 14:40:06 2021 +0300
ve: get_curr_ve: first try getting ve with rcu lock
By holding rcu lock we can have valid ve pointer. Next using css_tryget
we can get reference on ve cgroup if it is not yet started to destroy.
In case cgroup is destroying retry with cgroup_mutex.
https://jira.sw.ru/browse/PSBM-123766
Signed-off-by: Pavel Tikhomirov <[email protected]>
---
kernel/cgroup.c | 27 ++++++++++++++++++++++++---
kernel/ve/ve.c | 2 +-
2 files changed, 25 insertions(+), 4 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8f01cb9..846e226 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4655,10 +4655,31 @@ struct ve_struct *get_curr_ve(void)
struct ve_struct *ve;
/*
- * Under cgroup_mutex both current tasks ve cgroup and ->task_ve
- * pointer can't change. Corresponding cgroup_mutex around
- * cgroup_attach_task() protects us from it.
+ * If first thread loads current->task_ve pointer, and if just after
+ * that current is moved by other thread from this ve cgroup to some
+ * other and this ve cgroup gets destroyed, ve pointer gets freed, so
+ * first thread can't use such ve pointer safely.
+ */
+
+ /*
+ * Fast path: Let's make it safe with rcu lock, though current can be
+ * moved to other ve cgroup and our ve cgroup can start destroying, ve
+ * pointer would be still valid. As it is freed in ve_destroy. And
+ * ve_destroy is called from rcu callback after task_ve had changed.
*/
+ rcu_read_lock();
+ ve = rcu_dereference(current->task_ve);
+ if (css_tryget(&ve->css)) {
+ rcu_read_unlock();
+ return ve;
+ }
+ rcu_read_unlock();
+
+ /*
+ * Slow path: Under cgroup_mutex both current tasks ve cgroup and
+ * task_ve pointer can't change. Corresponding cgroup_mutex around
+ * cgroup_attach_task() protects us from it.
+ */
mutex_lock(&cgroup_mutex);
ve = get_ve(current->task_ve);
mutex_unlock(&cgroup_mutex);
diff --git a/kernel/ve/ve.c b/kernel/ve/ve.c
index 4480476..68c3e91 100644
--- a/kernel/ve/ve.c
+++ b/kernel/ve/ve.c
@@ -1099,7 +1099,7 @@ static void ve_attach(struct cgroup *cg, struct
cgroup_taskset *tset)
ve_try_set_task_start_time(ve, task);
- task->task_ve = ve;
+ rcu_assign_pointer(task->task_ve, ve);
}
/* Adjust cpuid faulting */
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