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cgroup: Make operations on the cgroup root_list RCU safe

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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@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Yafang Shao 2023-10-29 06:14:29 +00:00 committed by Tejun Heo
parent 96a2b48e5e
commit d23b5c5777
3 changed files with 19 additions and 8 deletions
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1
include/linux/cgroup-defs.h
View file

@ -563,6 +563,7 @@ 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;

3
kernel/cgroup/cgroup-internal.h
View file

@ -170,7 +170,8 @@ extern struct list_head cgroup_roots;
/* 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

23
kernel/cgroup/cgroup.c
View file

@ -1315,7 +1315,7 @@ 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)
@ -1348,7 +1348,7 @@ 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--;
if (!have_favordynmods)
@ -1389,7 +1389,15 @@ 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;
}
@ -1448,7 +1456,6 @@ 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);
@ -1456,7 +1463,9 @@ 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)
@ -2031,7 +2040,7 @@ 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);
@ -2114,7 +2123,7 @@ 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++;
/*