At present, when we perform operations on the cgroup root_list, we must
hold the cgroup_mutex, which is a relatively heavyweight lock. In reality,
we can make operations on this list RCU-safe, eliminating the need to hold
the cgroup_mutex during traversal. Modifications to the list only occur in
the cgroup root setup and destroy paths, which should be infrequent in a
production environment. In contrast, traversal may occur frequently.
Therefore, making it RCU-safe would be beneficial.
Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
---
include/linux/cgroup-defs.h | 1 +
kernel/cgroup/cgroup-internal.h | 3 ++-
kernel/cgroup/cgroup.c | 23 ++++++++++++++++-------
3 files changed, 19 insertions(+), 8 deletions(-)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index f1b3151..8505eea 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -558,6 +558,7 @@ struct cgroup_root {
/* A list running through the active hierarchies */
struct list_head root_list;
+ struct rcu_head rcu;
/* Hierarchy-specific flags */
unsigned int flags;
diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index c56071f..5e17f01 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -170,7 +170,8 @@ struct cgroup_mgctx {
/* iterate across the hierarchies */
#define for_each_root(root) \
- list_for_each_entry((root), &cgroup_roots, root_list)
+ list_for_each_entry_rcu((root), &cgroup_roots, root_list, \
+ lockdep_is_held(&cgroup_mutex))
/**
* for_each_subsys - iterate all enabled cgroup subsystems
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 3053d42..28b8ccc 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1313,7 +1313,7 @@ static void cgroup_exit_root_id(struct cgroup_root *root)
void cgroup_free_root(struct cgroup_root *root)
{
- kfree(root);
+ kfree_rcu(root, rcu);
}
static void cgroup_destroy_root(struct cgroup_root *root)
@@ -1346,7 +1346,7 @@ static void cgroup_destroy_root(struct cgroup_root *root)
spin_unlock_irq(&css_set_lock);
WARN_ON_ONCE(list_empty(&root->root_list));
- list_del(&root->root_list);
+ list_del_rcu(&root->root_list);
cgroup_root_count--;
cgroup_favor_dynmods(root, false);
@@ -1385,7 +1385,15 @@ static inline struct cgroup *__cset_cgroup_from_root(struct css_set *cset,
}
}
- BUG_ON(!res_cgroup);
+ /*
+ * If cgroup_mutex is not held, the cgrp_cset_link will be freed
+ * before we remove the cgroup root from the root_list. Consequently,
+ * when accessing a cgroup root, the cset_link may have already been
+ * freed, resulting in a NULL res_cgroup. However, by holding the
+ * cgroup_mutex, we ensure that res_cgroup can't be NULL.
+ * If we don't hold cgroup_mutex in the caller, we must do the NULL
+ * check.
+ */
return res_cgroup;
}
@@ -1444,7 +1452,6 @@ static struct cgroup *current_cgns_cgroup_dfl(void)
static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
struct cgroup_root *root)
{
- lockdep_assert_held(&cgroup_mutex);
lockdep_assert_held(&css_set_lock);
return __cset_cgroup_from_root(cset, root);
@@ -1452,7 +1459,9 @@ static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
/*
* Return the cgroup for "task" from the given hierarchy. Must be
- * called with cgroup_mutex and css_set_lock held.
+ * called with css_set_lock held to prevent task's groups from being modified.
+ * Must be called with either cgroup_mutex or rcu read lock to prevent the
+ * cgroup root from being destroyed.
*/
struct cgroup *task_cgroup_from_root(struct task_struct *task,
struct cgroup_root *root)
@@ -2013,7 +2022,7 @@ void init_cgroup_root(struct cgroup_fs_context *ctx)
struct cgroup_root *root = ctx->root;
struct cgroup *cgrp = &root->cgrp;
- INIT_LIST_HEAD(&root->root_list);
+ INIT_LIST_HEAD_RCU(&root->root_list);
atomic_set(&root->nr_cgrps, 1);
cgrp->root = root;
init_cgroup_housekeeping(cgrp);
@@ -2096,7 +2105,7 @@ int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
* care of subsystems' refcounts, which are explicitly dropped in
* the failure exit path.
*/
- list_add(&root->root_list, &cgroup_roots);
+ list_add_rcu(&root->root_list, &cgroup_roots);
cgroup_root_count++;
/*
--
1.8.3.1
