Commit 14611e51 authored by Tejun Heo's avatar Tejun Heo
Browse files

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: default avatarTejun Heo <tj@kernel.org>
Reported-by: default avatarFengguang Wu <fengguang.wu@intel.com>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
Cc: stable@vger.kernel.org
parent eb178d06

include/linux/cgroup.h

+48 −10
Original line number Diff line number Diff line
@@ -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)

{

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