diff options
| -rw-r--r-- | include/linux/llist.h | 6 | ||||
| -rw-r--r-- | kernel/cgroup/rstat.c | 28 |
2 files changed, 30 insertions, 4 deletions
diff --git a/include/linux/llist.h b/include/linux/llist.h index 27b17f64bcee..607b2360c938 100644 --- a/include/linux/llist.h +++ b/include/linux/llist.h @@ -83,7 +83,7 @@ static inline void init_llist_head(struct llist_head *list) */ static inline void init_llist_node(struct llist_node *node) { - node->next = node; + WRITE_ONCE(node->next, node); } /** @@ -97,7 +97,7 @@ static inline void init_llist_node(struct llist_node *node) */ static inline bool llist_on_list(const struct llist_node *node) { - return node->next != node; + return READ_ONCE(node->next) != node; } /** @@ -220,7 +220,7 @@ static inline bool llist_empty(const struct llist_head *head) static inline struct llist_node *llist_next(struct llist_node *node) { - return node->next; + return READ_ONCE(node->next); } /** diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c index c8a48cf83878..981e2f77ad4e 100644 --- a/kernel/cgroup/rstat.c +++ b/kernel/cgroup/rstat.c @@ -60,6 +60,12 @@ static inline struct llist_head *ss_lhead_cpu(struct cgroup_subsys *ss, int cpu) * Atomically inserts the css in the ss's llist for the given cpu. This is * reentrant safe i.e. safe against softirq, hardirq and nmi. The ss's llist * will be processed at the flush time to create the update tree. + * + * NOTE: if the user needs the guarantee that the updater either add itself in + * the lockless list or the concurrent flusher flushes its updated stats, a + * memory barrier is needed before the call to css_rstat_updated() i.e. a + * barrier after updating the per-cpu stats and before calling + * css_rstat_updated(). */ __bpf_kfunc void css_rstat_updated(struct cgroup_subsys_state *css, int cpu) { @@ -86,7 +92,12 @@ __bpf_kfunc void css_rstat_updated(struct cgroup_subsys_state *css, int cpu) return; rstatc = css_rstat_cpu(css, cpu); - /* If already on list return. */ + /* + * If already on list return. This check is racy and smp_mb() is needed + * to pair it with the smp_mb() in css_process_update_tree() if the + * guarantee that the updated stats are visible to concurrent flusher is + * needed. + */ if (llist_on_list(&rstatc->lnode)) return; @@ -148,6 +159,21 @@ static void css_process_update_tree(struct cgroup_subsys *ss, int cpu) while ((lnode = llist_del_first_init(lhead))) { struct css_rstat_cpu *rstatc; + /* + * smp_mb() is needed here (more specifically in between + * init_llist_node() and per-cpu stats flushing) if the + * guarantee is required by a rstat user where etiher the + * updater should add itself on the lockless list or the + * flusher flush the stats updated by the updater who have + * observed that they are already on the list. The + * corresponding barrier pair for this one should be before + * css_rstat_updated() by the user. + * + * For now, there aren't any such user, so not adding the + * barrier here but if such a use-case arise, please add + * smp_mb() here. + */ + rstatc = container_of(lnode, struct css_rstat_cpu, lnode); __css_process_update_tree(rstatc->owner, cpu); } |