On Tue, 2010-06-08 at 00:16 -0400, Miles Lane wrote:

quoted

On Mon, Jun 7, 2010 at 8:19 PM, Paul E. McKenney
[off-list ref] wrote:

quoted

On Mon, Jun 07, 2010 at 02:14:25PM -0400, Miles Lane wrote:

quoted

Hi All,

I just reproduced a warning I reported quite a while ago.  Is a patch
for this in the pipeline?

I proposed a patch, thinking that it was a false positive.  Peter Zijlstra
pointed out that there was a real race, and proposed an alternative patch,
which may be found at http://lkml.org/lkml/2010/4/22/603.

Could you please test Peter's patch and let us know if it cures the problem?

Gah, this task_group() stuff is annoying, how about something like the
below which teaches task_group() about the task_rq()->lock rule?

---
 include/linux/cgroup.h |   20 +++++++++++----
 kernel/sched.c         |   61 +++++++++++++++++++++++++----------------------
 2 files changed, 46 insertions(+), 35 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 0c62160..1efd212 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h

@@ -525,13 +525,21 @@ static inline struct cgroup_subsys_state *cgroup_subsys_state(

       return cgrp->subsys[subsys_id];
 }

-static inline struct cgroup_subsys_state *task_subsys_state(
-       struct task_struct *task, int 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.
+ */
+#define task_subsys_state_check(task, subsys_id, __c)                  \
+       rcu_dereference_check(task->cgroups->subsys[subsys_id],         \
+                             rcu_read_lock_held() ||                   \
+                             lockdep_is_held(&task->alloc_lock) ||     \
+                             cgroup_lock_is_held() || (__c))
+
+static inline struct cgroup_subsys_state *
+task_subsys_state(struct task_struct *task, int subsys_id)
 {
-       return rcu_dereference_check(task->cgroups->subsys[subsys_id],
-                                    rcu_read_lock_held() ||
-                                    lockdep_is_held(&task->alloc_lock) ||
-                                    cgroup_lock_is_held());
+       return task_subsys_state_check(task, subsys_id, false);
 }

 static inline struct cgroup* task_cgroup(struct task_struct *task,

diff --git a/kernel/sched.c b/kernel/sched.c
index f8b8996..e01bb45 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c

@@ -306,32 +306,26 @@ static int init_task_group_load = INIT_TASK_GROUP_LOAD;

 */
 struct task_group init_task_group;

-/* return group to which a task belongs */
+/*
+ * Return the group to which this tasks belongs.
+ *
+ * We use task_subsys_state_check() and extend the RCU verification
+ * with lockdep_is_held(&task_rq(p)->lock) because cpu_cgroup_attach()
+ * holds that lock for each task it moves into the cgroup. Therefore
+ * by holding that lock, we pin the task to the current cgroup.
+ */
 static inline struct task_group *task_group(struct task_struct *p)
 {
-       struct task_group *tg;
+       struct cgroup_subsys_state *css;

-#ifdef CONFIG_CGROUP_SCHED
-       tg = container_of(task_subsys_state(p, cpu_cgroup_subsys_id),
-                               struct task_group, css);
-#else
-       tg = &init_task_group;
-#endif
-       return tg;
+       css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
+                       lockdep_is_held(&task_rq(p)->lock));
+       return container_of(css, struct task_group, css);
 }

 /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
 static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
 {
-       /*
-        * Strictly speaking this rcu_read_lock() is not needed since the
-        * task_group is tied to the cgroup, which in turn can never go away
-        * as long as there are tasks attached to it.
-        *
-        * However since task_group() uses task_subsys_state() which is an
-        * rcu_dereference() user, this quiets CONFIG_PROVE_RCU.
-        */
-       rcu_read_lock();
 #ifdef CONFIG_FAIR_GROUP_SCHED
       p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
       p->se.parent = task_group(p)->se[cpu];

@@ -341,7 +335,6 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)

       p->rt.rt_rq  = task_group(p)->rt_rq[cpu];
       p->rt.parent = task_group(p)->rt_se[cpu];
 #endif
-       rcu_read_unlock();
 }

 #else

@@ -4465,16 +4458,6 @@ recheck:

       }

       if (user) {
-#ifdef CONFIG_RT_GROUP_SCHED
-               /*
-                * Do not allow realtime tasks into groups that have no runtime
-                * assigned.
-                */
-               if (rt_bandwidth_enabled() && rt_policy(policy) &&
-                               task_group(p)->rt_bandwidth.rt_runtime == 0)
-                       return -EPERM;
-#endif
-
               retval = security_task_setscheduler(p, policy, param);
               if (retval)
                       return retval;

@@ -4490,6 +4473,26 @@ recheck:

        * runqueue lock must be held.
        */
       rq = __task_rq_lock(p);
+
+       retval = 0;
+#ifdef CONFIG_RT_GROUP_SCHED
+       if (user) {
+               /*
+                * Do not allow realtime tasks into groups that have no runtime
+                * assigned.
+                */
+               if (rt_bandwidth_enabled() && rt_policy(policy) &&
+                               task_group(p)->rt_bandwidth.rt_runtime == 0)
+                       retval = -EPERM;
+
+               if (retval) {
+                       __task_rq_unlock(rq);
+                       raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+                       return retval;
+               }
+       }
+#endif
+
       /* recheck policy now with rq lock held */
       if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) {
               policy = oldpolicy = -1;