Merge branch 'core-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip

* 'core-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
  rcu: Fix RCU lockdep splat on freezer_fork path
  rcu: Fix RCU lockdep splat in set_task_cpu on fork path
  mutex: Don't spin when the owner CPU is offline or other weird cases

2 files changed, 18 insertions, 5 deletions

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index da5e13975531..e5c0244962b0 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -205,9 +205,12 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
          * No lock is needed, since the task isn't on tasklist yet,
          * so it can't be moved to another cgroup, which means the
          * freezer won't be removed and will be valid during this
-         * function call.
+         * function call.  Nevertheless, apply RCU read-side critical
+         * section to suppress RCU lockdep false positives.
          */
+        rcu_read_lock();
         freezer = task_freezer(task);
+        rcu_read_unlock();
 
         /*
          * The root cgroup is non-freezable, so we can skip the
diff --git a/kernel/sched.c b/kernel/sched.c
index 6af210a7de70..3c2a54f70ffe 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -323,6 +323,15 @@ static inline struct task_group *task_group(struct task_struct *p)
 /* 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];
@@ -332,6 +341,7 @@ 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
@@ -3780,7 +3790,7 @@ int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
          * the mutex owner just released it and exited.
          */
         if (probe_kernel_address(&owner->cpu, cpu))
-                goto out;
+                return 0;
 #else
         cpu = owner->cpu;
 #endif
@@ -3790,14 +3800,14 @@ int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
          * the cpu field may no longer be valid.
          */
         if (cpu >= nr_cpumask_bits)
-                goto out;
+                return 0;
 
         /*
          * We need to validate that we can do a
          * get_cpu() and that we have the percpu area.
          */
         if (!cpu_online(cpu))
-                goto out;
+                return 0;
 
         rq = cpu_rq(cpu);
 
@@ -3816,7 +3826,7 @@ int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
 
                 cpu_relax();
         }
-out:
+
         return 1;
 }
 #endif