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authorTejun Heo <tj@kernel.org>2013-06-25 11:48:32 -0700
committerBen Hutchings <ben@decadent.org.uk>2013-07-27 05:34:20 +0100
commit2235df8b82f6333f2f1a4c6e0acf80f19f591b55 (patch)
treec537c3f631e13b939d3cb8e8207e68306b0f83f8
parentbe45f4eba5d9c264dacd10dee0ba23ec14624911 (diff)
downloadlwn-2235df8b82f6333f2f1a4c6e0acf80f19f591b55.tar.gz
lwn-2235df8b82f6333f2f1a4c6e0acf80f19f591b55.zip
cgroup: fix RCU accesses to task->cgroups
commit 14611e51a57df10240817d8ada510842faf0ec51 upstream. 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 <tj@kernel.org> Reported-by: Fengguang Wu <fengguang.wu@intel.com> Acked-by: Li Zefan <lizefan@huawei.com> [bwh: Backported to 3.2: - Adjust context - Remove CONFIG_PROVE_RCU condition - s/lockdep_is_held(&cgroup_mutex)/cgroup_lock_is_held()/] Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
-rw-r--r--include/linux/cgroup.h52
1 files changed, 45 insertions, 7 deletions
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 9bab75f56912..ace098455ee0 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -531,16 +531,54 @@ 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.
+ */
+#define task_css_set_check(task, __c) \
+ rcu_dereference_check((task)->cgroups, \
+ lockdep_is_held(&(task)->alloc_lock) || \
+ cgroup_lock_is_held() || (__c))
+
+/**
+ * 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) \
- rcu_dereference_check(task->cgroups->subsys[subsys_id], \
- lockdep_is_held(&task->alloc_lock) || \
- cgroup_lock_is_held() || (__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)
{