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cgroup: fix RCU accesses to task->cgroups

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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>
Cc: stable@vger.kernel.org
This commit is contained in:
Tejun Heo 2013-06-25 11:48:32 -07:00
commit 14611e51a5
1 changed files with 48 additions and 10 deletions
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54
include/linux/cgroup.h
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@ -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)], \
#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)
{