sched, cgroup: reorganize threadgroup locking

threadgroup_change_begin/end() are used to mark the beginning and end
of threadgroup modifying operations to allow code paths which require
a threadgroup to stay stable across blocking operations to synchronize
against those sections using threadgroup_lock/unlock().

It's currently implemented as a general mechanism in sched.h using
per-signal_struct rwsem; however, this never grew non-cgroup use cases
and becomes noop if !CONFIG_CGROUPS.  It turns out that cgroups is
gonna be better served with a different sycnrhonization scheme and is
a bit silly to keep cgroups specific details as a general mechanism.

What's general here is identifying the places where threadgroups are
modified.  This patch restructures threadgroup locking so that
threadgroup_change_begin/end() become a place where subsystems which
need to sycnhronize against threadgroup changes can hook into.

cgroup_threadgroup_change_begin/end() which operate on the
per-signal_struct rwsem are created and threadgroup_lock/unlock() are
moved to cgroup.c and made static.

This is pure reorganization which doesn't cause any functional
changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

include/linux/cgroup-defs.h

@@ -14,6 +14,7 @@
 #include <linux/mutex.h>
 #include <linux/rcupdate.h>
 #include <linux/percpu-refcount.h>
+#include <linux/percpu-rwsem.h>
 #include <linux/workqueue.h>
 
 #ifdef CONFIG_CGROUPS
@@ -460,5 +461,14 @@ struct cgroup_subsys {
 	unsigned int depends_on;
 };
 
+void cgroup_threadgroup_change_begin(struct task_struct *tsk);
+void cgroup_threadgroup_change_end(struct task_struct *tsk);
+
+#else	/* CONFIG_CGROUPS */
+
+static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk) {}
+static inline void cgroup_threadgroup_change_end(struct task_struct *tsk) {}
+
 #endif	/* CONFIG_CGROUPS */
+
 #endif	/* _LINUX_CGROUP_DEFS_H */