When memcg code needs to know whether any given memcg has children, it uses the cgroup child iteration primitives and returns true/false depending on whether the iteration loop is executed at least once or not. Because a cgroup's list of children is RCU protected, these primitives require the RCU read-lock to be held, which is not the case for all memcg callers. This results in the following splat when e.g. enabling hierarchy mode: WARNING: CPU: 3 PID: 1 at kernel/cgroup.c:3043 css_next_child+0xa3/0x160() CPU: 3 PID: 1 Comm: systemd Not tainted 3.12.0-rc5-00117-g83f11a9-dirty #18 Hardware name: LENOVO 3680B56/3680B56, BIOS 6QET69WW (1.39 ) 04/26/2012 Call Trace: dump_stack+0x54/0x74 warn_slowpath_common+0x78/0xa0 warn_slowpath_null+0x1a/0x20 css_next_child+0xa3/0x160 mem_cgroup_hierarchy_write+0x5b/0xa0 cgroup_file_write+0x108/0x2a0 vfs_write+0xbd/0x1e0 SyS_write+0x4c/0xa0 system_call_fastpath+0x16/0x1b In the memcg case, we only care about children when we are attempting to modify inheritable attributes interactively. Racing with deletion could mean a spurious -EBUSY, no problem. Racing with addition is handled just fine as well through the memcg_create_mutex: if the child group is not on the list after the mutex is acquired, it won't be initialized from the parent's attributes until after the unlock.
Unified · Split
| @@ -4959,31 +4959,18 @@ static void mem_cgroup_reparent_charges(struct mem_cgroup *memcg) · 4958 unmodified lines | ||
| 4959 | 4959 | } while (usage > 0); |
| 4960 | 4960 | } |
| 4961 | 4961 | |
| 4962 | −/* | |
| 4963 | − * This mainly exists for tests during the setting of set of use_hierarchy. | |
| 4964 | − * Since this is the very setting we are changing, the current hierarchy value | |
| 4965 | − * is meaningless | |
| 4966 | − */ | |
| 4967 | −static inline bool __memcg_has_children(struct mem_cgroup *memcg) | |
| 4968 | −{ | |
| 4969 | − struct cgroup_subsys_state *pos; | |
| 4970 | − | |
| 4971 | − /* bounce at first found */ | |
| 4972 | − css_for_each_child(pos, &memcg->css) | |
| 4973 | − return true; | |
| 4974 | − return false; | |
| 4975 | −} | |
| 4976 | − | |
| 4977 | −/* | |
| 4978 | − * Must be called with memcg_create_mutex held, unless the cgroup is guaranteed | |
| 4979 | − * to be already dead (as in mem_cgroup_force_empty, for instance). This is | |
| 4980 | − * from mem_cgroup_count_children(), in the sense that we don't really care how | |
| 4981 | − * many children we have; we only need to know if we have any. It also counts | |
| 4982 | − * any memcg without hierarchy as infertile. | |
| 4983 | − */ | |
| 4984 | 4962 | static inline bool memcg_has_children(struct mem_cgroup *memcg) |
| 4985 | 4963 | { |
| 4986 | − return memcg->use_hierarchy && __memcg_has_children(memcg); | |
| 4964 | + lockdep_assert_held(&memcg_create_mutex); | |
| 4965 | + /* | |
| 4966 | + * The lock does not prevent addition or deletion to the list | |
| 4967 | + * of children, but it prevents a new child from being | |
| 4968 | + * initialized based on this parent in css_online(), so it's | |
| 4969 | + * enough to decide whether hierarchically inherited | |
| 4970 | + * attributes can still be changed or not. | |
| 4971 | + */ | |
| 4972 | + return memcg->use_hierarchy && | |
| 4973 | + !list_empty(&memcg->css.cgroup->children); | |
| 4987 | 4974 | } |
| 4988 | 4975 | |
| 4989 | 4976 | /* |
| @@ -5063,7 +5050,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup_subsys_state *css, · 73 unmodified lines | ||
| 5063 | 5050 | */ |
| 5064 | 5051 | if ((!parent_memcg || !parent_memcg->use_hierarchy) && |
| 5065 | 5052 | (val == 1 || val == 0)) { |
| 5066 | − if (!__memcg_has_children(memcg)) | |
| 5053 | + if (list_empty(&memcg->css.cgroup->children)) | |
| 5067 | 5054 | memcg->use_hierarchy = val; |
| 5068 | 5055 | else |
| 5069 | 5056 | retval = -EBUSY; |