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

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

On 8/12/26 18:42, Konstantin Khorenko wrote:
> 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>

Reviewed-by: Pavel Tikhomirov <ptikhomirov@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) ||

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.