From 14611e51a57df10240817d8ada510842faf0ec51 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 25 Jun 2013 11:48:32 -0700 Subject: [PATCH] cgroup: fix RCU accesses to task->cgroups task->cgroups is a RCU pointer pointing to struct css_set. A task switches to a different css_set on cgroup migration but a css_set doesn't change once created and its pointers to cgroup_subsys_states aren't RCU protected. task_subsys_state[_check]() is the macro to acquire css given a task and subsys_id pair. It RCU-dereferences task->cgroups->subsys[] not task->cgroups, so the RCU pointer task->cgroups ends up being dereferenced without read_barrier_depends() after it. It's broken. Fix it by introducing task_css_set[_check]() which does RCU-dereference on task->cgroups. task_subsys_state[_check]() is reimplemented to directly dereference ->subsys[] of the css_set returned from task_css_set[_check](). This removes some of sparse RCU warnings in cgroup. v2: Fixed unbalanced parenthsis and there's no need to use rcu_dereference_raw() when !CONFIG_PROVE_RCU. Both spotted by Li. Signed-off-by: Tejun Heo Reported-by: Fengguang Wu Acked-by: Li Zefan Cc: stable@vger.kernel.org --- include/linux/cgroup.h | 58 ++++++++++++++++++++++++++++++++++-------- 1 file changed, 48 insertions(+), 10 deletions(-) diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h index 8e4fd5e67384..ad3555bc21f4 100644 --- a/include/linux/cgroup.h +++ b/include/linux/cgroup.h @@ -635,22 +635,60 @@ static inline struct cgroup_subsys_state *cgroup_subsys_state( return cgrp->subsys[subsys_id]; } -/* - * function to get the cgroup_subsys_state which allows for extra - * rcu_dereference_check() conditions, such as locks used during the - * cgroup_subsys::attach() methods. +/** + * task_css_set_check - obtain a task's css_set with extra access conditions + * @task: the task to obtain css_set for + * @__c: extra condition expression to be passed to rcu_dereference_check() + * + * A task's css_set is RCU protected, initialized and exited while holding + * task_lock(), and can only be modified while holding both cgroup_mutex + * and task_lock() while the task is alive. This macro verifies that the + * caller is inside proper critical section and returns @task's css_set. + * + * The caller can also specify additional allowed conditions via @__c, such + * as locks used during the cgroup_subsys::attach() methods. */ #ifdef CONFIG_PROVE_RCU extern struct mutex cgroup_mutex; -#define task_subsys_state_check(task, subsys_id, __c) \ - rcu_dereference_check((task)->cgroups->subsys[(subsys_id)], \ - lockdep_is_held(&(task)->alloc_lock) || \ - lockdep_is_held(&cgroup_mutex) || (__c)) +#define task_css_set_check(task, __c) \ + rcu_dereference_check((task)->cgroups, \ + lockdep_is_held(&(task)->alloc_lock) || \ + lockdep_is_held(&cgroup_mutex) || (__c)) #else -#define task_subsys_state_check(task, subsys_id, __c) \ - rcu_dereference((task)->cgroups->subsys[(subsys_id)]) +#define task_css_set_check(task, __c) \ + rcu_dereference((task)->cgroups) #endif +/** + * task_subsys_state_check - obtain css for (task, subsys) w/ extra access conds + * @task: the target task + * @subsys_id: the target subsystem ID + * @__c: extra condition expression to be passed to rcu_dereference_check() + * + * Return the cgroup_subsys_state for the (@task, @subsys_id) pair. The + * synchronization rules are the same as task_css_set_check(). + */ +#define task_subsys_state_check(task, subsys_id, __c) \ + task_css_set_check((task), (__c))->subsys[(subsys_id)] + +/** + * task_css_set - obtain a task's css_set + * @task: the task to obtain css_set for + * + * See task_css_set_check(). + */ +static inline struct css_set *task_css_set(struct task_struct *task) +{ + return task_css_set_check(task, false); +} + +/** + * task_subsys_state - obtain css for (task, subsys) + * @task: the target task + * @subsys_id: the target subsystem ID + * + * See task_subsys_state_check(). + */ static inline struct cgroup_subsys_state * task_subsys_state(struct task_struct *task, int subsys_id) { -- 2.39.5