[Devel] [PATCH vz10 v2] cgroup: use proper lockdep condition for ve_nsproxy dereference

[Konstantin Khorenko <khorenko@virtuozzo.com>]

cgroup_mark_ve_roots(), cgroup_unmark_ve_roots(), cgroup_join_vz_slice()
and cgroup_leave_vz_slice() dereference ve->ve_nsproxy with
rcu_dereference_protected(..., 1), which unconditionally silences lockdep
without naming the actual lock that makes the access safe.

All four are only called from ve_start_container() and ve_exit_ns(), both
of which run under ve->op_sem write-lock. The pointer is stable there
because ve_grab_context() publishes it via rcu_assign_pointer() and
ve_drop_context() - the only path that clears it - cannot run
concurrently since it also requires op_sem.

Replace the unconditional "1" with lockdep_is_held(&ve->op_sem) so that
lockdep can actually verify the locking at runtime. This is consistent
with how ve_grab_context(), ve_drop_context(), ve_stop_ns() and
ve_exit_ns() already annotate the same pointer. Store the result in a
local variable to keep the lines readable, and extend the existing
comment to mention op_sem.

No functional change: the condition is only evaluated under
CONFIG_PROVE_RCU.

https://virtuozzo.atlassian.net/browse/VSTOR-128317

Feature: ve: ve generic structures
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
 kernel/cgroup/cgroup.c | 54 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 44 insertions(+), 10 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 4db77090f1be1..948a2355df833 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2179,15 +2179,21 @@ static inline bool ve_check_root_cgroups(struct css_set *cset)
 int cgroup_mark_ve_roots(struct ve_struct *ve)
 {
 	struct cgrp_cset_link *link;
+	struct nsproxy *ve_nsproxy;
 	struct css_set *cset;
 	struct cgroup *cgrp;
 
 	/*
-	 * It's safe to use ve->ve_nsproxy->cgroup_ns->root_cset here without extra
-	 * locking as we do it from container init at container start after
-	 * ve_grab_context and only container init can tear those down.
+	 * It's safe to use ve->ve_nsproxy->cgroup_ns->root_cset here without
+	 * extra locking as we do it from container init at container start
+	 * after ve_grab_context and only container init can tear those down.
+	 * Still ve->op_sem is held here which serializes ve->ve_nsproxy usage
+	 * against ve_drop_context(), so let lockdep verify the context without
+	 * external knowledge.
 	 */
-	cset = rcu_dereference_protected(ve->ve_nsproxy, 1)->cgroup_ns->root_cset;
+	ve_nsproxy = rcu_dereference_protected(ve->ve_nsproxy,
+					       lockdep_is_held(&ve->op_sem));
+	cset = ve_nsproxy->cgroup_ns->root_cset;
 	BUG_ON(!cset);
 
 	spin_lock_irq(&css_set_lock);
@@ -2217,15 +2223,21 @@ int cgroup_mark_ve_roots(struct ve_struct *ve)
 void cgroup_unmark_ve_roots(struct ve_struct *ve)
 {
 	struct cgrp_cset_link *link;
+	struct nsproxy *ve_nsproxy;
 	struct css_set *cset;
 	struct cgroup *cgrp;
 
 	/*
-	 * It's safe to use ve->ve_nsproxy->cgroup_ns->root_cset here without extra
-	 * locking as we do it from container init at container start after
-	 * ve_grab_context and only container init can tear those down.
+	 * It's safe to use ve->ve_nsproxy->cgroup_ns->root_cset here without
+	 * extra locking as we do it from container init at container start
+	 * after ve_grab_context and only container init can tear those down.
+	 * Still ve->op_sem is held here which serializes ve->ve_nsproxy usage
+	 * against ve_drop_context(), so let lockdep verify the context without
+	 * external knowledge.
 	 */
-	cset = rcu_dereference_protected(ve->ve_nsproxy, 1)->cgroup_ns->root_cset;
+	ve_nsproxy = rcu_dereference_protected(ve->ve_nsproxy,
+					       lockdep_is_held(&ve->op_sem));
+	cset = ve_nsproxy->cgroup_ns->root_cset;
 	BUG_ON(!cset);
 
 	spin_lock_irq(&css_set_lock);
@@ -2247,12 +2259,23 @@ void cgroup_unmark_ve_roots(struct ve_struct *ve)
 
 int cgroup_join_vz_slice(struct ve_struct *ve)
 {
+	struct nsproxy *ve_nsproxy;
 	struct kernfs_node *kn;
 	struct css_set *cset;
 	struct cgroup *cgrp;
 	int ret;
 
-	cset = rcu_dereference_protected(ve->ve_nsproxy, 1)->cgroup_ns->root_cset;
+	/*
+	 * It's safe to use ve->ve_nsproxy->cgroup_ns->root_cset here without
+	 * extra locking as we do it from container init at container start
+	 * after ve_grab_context and only container init can tear those down.
+	 * Still ve->op_sem is held here which serializes ve->ve_nsproxy usage
+	 * against ve_drop_context(), so let lockdep verify the context without
+	 * external knowledge.
+	 */
+	ve_nsproxy = rcu_dereference_protected(ve->ve_nsproxy,
+					       lockdep_is_held(&ve->op_sem));
+	cset = ve_nsproxy->cgroup_ns->root_cset;
 	cgrp = __cset_cgroup_from_root(cset, &cgrp_dfl_root);
 
 	if (!is_virtualized_cgroup(cgrp) ||
@@ -2286,11 +2309,22 @@ int cgroup_join_vz_slice(struct ve_struct *ve)
 
 int cgroup_leave_vz_slice(struct ve_struct *ve)
 {
+	struct nsproxy *ve_nsproxy;
 	struct css_set *cset;
 	struct cgroup *cgrp;
 	int ret;
 
-	cset = rcu_dereference_protected(ve->ve_nsproxy, 1)->cgroup_ns->root_cset;
+	/*
+	 * It's safe to use ve->ve_nsproxy->cgroup_ns->root_cset here without
+	 * extra locking as we do it from container init at container start
+	 * after ve_grab_context and only container init can tear those down.
+	 * Still ve->op_sem is held here which serializes ve->ve_nsproxy usage
+	 * against ve_drop_context(), so let lockdep verify the context without
+	 * external knowledge.
+	 */
+	ve_nsproxy = rcu_dereference_protected(ve->ve_nsproxy,
+					       lockdep_is_held(&ve->op_sem));
+	cset = ve_nsproxy->cgroup_ns->root_cset;
 	cgrp = __cset_cgroup_from_root(cset, &cgrp_dfl_root);
 
 	if (!is_virtualized_cgroup(cgrp) ||
-- 
2.43.0