[RFC PATCH 3/3] cgroup: Do not take css_set_lock in cgroup_show_path

[Michal Koutný]

/proc/$pid/mountinfo may accumulate lots of entries (causing frequent
re-reads of whole file) or lots cgroupfs entries alone.
The cgroupfs entries rendered with cgroup_show_path() may increase/be
subject of css_set_lock contention causing further slowdown -- not only
mountinfo rendering but any other css_set_lock user.

We leverage the fact that mountinfo reading happens with namespace_sem
taken and hierarchy roots thus cannot be freed concurrently.

There are three relevant nodes for each cgroupfs entry:

        R ... cgroup hierarchy root
        M ... mount root
        C ... reader's cgroup NS root

mountinfo is supposed to show path from C to M.

Current's css_set (and linked root cgroups) are stable under
namespace_sem, therefore current_cgns_cgroup_from_root() doesn't need
css_set_lock.

When the path is assembled in kernfs_path_from_node(), we know that:
- C kernfs_node is (transitively) pinned via current->nsproxy,
- M kernfs_node is pinned thanks to namespace_sem,
- path C to M is pinned via child->parent references (this holds also
  when C and M are in distinct subtrees).

Streamline mountinfo rendering a bit by relieving css_set_lock and add
careful notes about that.

Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
---
 kernel/cgroup/cgroup.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 32d693a797b9..e2ec6f0028be 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1407,12 +1407,18 @@ static inline struct cgroup *__cset_cgroup_from_root(struct css_set *cset,
 	struct cgroup *res_cgroup = NULL;
 
 	if (cset == &init_css_set) {
+		/* callers must ensure root stability */
 		res_cgroup = &root->cgrp;
 	} else if (root == &cgrp_dfl_root) {
 		res_cgroup = cset->dfl_cgrp;
 	} else {
 		struct cgrp_cset_link *link;
-		lockdep_assert_held(&css_set_lock);
+		/* cset's cgroups are pinned unless they are root cgroups that
+		 * were unmounted.  We look at links to !cgrp_dfl_root
+		 * cgroup_root, either lock ensures the list is not mutated
+		 */
+		lockdep_assert(lockdep_is_held(&css_set_lock) ||
+			       lockdep_is_held_type(&namespace_sem, -1));
 
 		list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
 			struct cgroup *c = link->cgrp;
@@ -1438,8 +1444,6 @@ current_cgns_cgroup_from_root(struct cgroup_root *root)
 	struct cgroup *res = NULL;
 	struct css_set *cset;
 
-	lockdep_assert_held(&css_set_lock);
-
 	/* namespace_sem ensures `root` stability on unmount */
 	lockdep_assert(lockdep_is_held_type(&namespace_sem, -1));
 
@@ -1905,10 +1909,8 @@ int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
 	if (!buf)
 		return -ENOMEM;
 
-	spin_lock_irq(&css_set_lock);
 	ns_cgroup = current_cgns_cgroup_from_root(kf_cgroot);
 	len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX);
-	spin_unlock_irq(&css_set_lock);
 
 	if (len >= PATH_MAX)
 		len = -ERANGE;
-- 
2.40.1