sched: Fix race in task_group()

Stefan reported a crash on a kernel before a3e5d1091c1 ("sched:
Don't call task_group() too many times in set_task_rq()"), he
found the reason to be that the multiple task_group()
invocations in set_task_rq() returned different values.

Looking at all that I found a lack of serialization and plain
wrong comments.

The below tries to fix it using an extra pointer which is
updated under the appropriate scheduler locks. Its not pretty,
but I can't really see another way given how all the cgroup
stuff works.

Reported-and-tested-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1340364965.18025.71.camel@twins
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 9e65eff..b806b82 100644 (file)
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -123,8 +123,17 @@ extern struct group_info init_groups;
 
 extern struct cred init_cred;
 
+extern struct task_group root_task_group;
+
+#ifdef CONFIG_CGROUP_SCHED
+# define INIT_CGROUP_SCHED(tsk)                                                \
+       .sched_task_group = &root_task_group,
+#else
+# define INIT_CGROUP_SCHED(tsk)
+#endif
+
 #ifdef CONFIG_PERF_EVENTS
-# define INIT_PERF_EVENTS(tsk)                                 \
+# define INIT_PERF_EVENTS(tsk)                                         \
        .perf_event_mutex =                                             \
                 __MUTEX_INITIALIZER(tsk.perf_event_mutex),             \
        .perf_event_list = LIST_HEAD_INIT(tsk.perf_event_list),
@@ -161,6 +170,7 @@ extern struct cred init_cred;
        },                                                              \
        .tasks          = LIST_HEAD_INIT(tsk.tasks),                    \
        INIT_PUSHABLE_TASKS(tsk)                                        \
+       INIT_CGROUP_SCHED(tsk)                                          \
        .ptraced        = LIST_HEAD_INIT(tsk.ptraced),                  \
        .ptrace_entry   = LIST_HEAD_INIT(tsk.ptrace_entry),             \
        .real_parent    = &tsk,                                         \

diff --git a/include/linux/sched.h b/include/linux/sched.h
index bc99529..fd9436a 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1245,6 +1245,9 @@ struct task_struct {
        const struct sched_class *sched_class;
        struct sched_entity se;
        struct sched_rt_entity rt;
+#ifdef CONFIG_CGROUP_SCHED
+       struct task_group *sched_task_group;
+#endif
 
 #ifdef CONFIG_PREEMPT_NOTIFIERS
        /* list of struct preempt_notifier: */
@@ -2724,7 +2727,7 @@ extern int sched_group_set_rt_period(struct task_group *tg,
 extern long sched_group_rt_period(struct task_group *tg);
 extern int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk);
 #endif
-#endif
+#endif /* CONFIG_CGROUP_SCHED */
 
 extern int task_can_switch_user(struct user_struct *up,
                                        struct task_struct *tsk);

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 536b213..5d011ef 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1096,7 +1096,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
         * a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks.
         *
         * sched_move_task() holds both and thus holding either pins the cgroup,
-        * see set_task_rq().
+        * see task_group().
         *
         * Furthermore, all task_rq users should acquire both locks, see
         * task_rq_lock().
@@ -7658,6 +7658,7 @@ void sched_destroy_group(struct task_group *tg)
  */
 void sched_move_task(struct task_struct *tsk)
 {
+       struct task_group *tg;
        int on_rq, running;
        unsigned long flags;
        struct rq *rq;
@@ -7672,6 +7673,12 @@ void sched_move_task(struct task_struct *tsk)
        if (unlikely(running))
                tsk->sched_class->put_prev_task(rq, tsk);
 
+       tg = container_of(task_subsys_state_check(tsk, cpu_cgroup_subsys_id,
+                               lockdep_is_held(&tsk->sighand->siglock)),
+                         struct task_group, css);
+       tg = autogroup_task_group(tsk, tg);
+       tsk->sched_task_group = tg;
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
        if (tsk->sched_class->task_move_group)
                tsk->sched_class->task_move_group(tsk, on_rq);

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 55844f2..c35a1a7 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -538,22 +538,19 @@ extern int group_balance_cpu(struct sched_group *sg);
 /*
  * Return the group to which this tasks belongs.
  *
- * We use task_subsys_state_check() and extend the RCU verification with
- * pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each
- * task it moves into the cgroup. Therefore by holding either of those locks,
- * we pin the task to the current cgroup.
+ * We cannot use task_subsys_state() and friends because the cgroup
+ * subsystem changes that value before the cgroup_subsys::attach() method
+ * is called, therefore we cannot pin it and might observe the wrong value.
+ *
+ * The same is true for autogroup's p->signal->autogroup->tg, the autogroup
+ * core changes this before calling sched_move_task().
+ *
+ * Instead we use a 'copy' which is updated from sched_move_task() while
+ * holding both task_struct::pi_lock and rq::lock.
  */
 static inline struct task_group *task_group(struct task_struct *p)
 {
-       struct task_group *tg;
-       struct cgroup_subsys_state *css;
-
-       css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
-                       lockdep_is_held(&p->pi_lock) ||
-                       lockdep_is_held(&task_rq(p)->lock));
-       tg = container_of(css, struct task_group, css);
-
-       return autogroup_task_group(p, tg);
+       return p->sched_task_group;
 }
 
 /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */