cgroup: make css_set_rwsem a spinlock and rename it to css_set_lock

css_set_rwsem is the inner lock protecting css_sets and is accessed
from hot paths such as fork and exit.  Internally, it has no reason to
be a rwsem or even mutex.  There are no internal blocking operations
while holding it.  This was rwsem because css task iteration used to
expose it to external iterator users.  As the previous patch updated
css task iteration such that the locking is not leaked to its users,
there's no reason to keep it a rwsem.

This patch converts css_set_rwsem to a spinlock and rename it to
css_set_lock.  It uses bh-safe operations as a planned usage needs to
access it from RCU callback context.
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -13,7 +13,6 @@
 #include <linux/nodemask.h>
 #include <linux/rculist.h>
 #include <linux/cgroupstats.h>
-#include <linux/rwsem.h>
 #include <linux/fs.h>
 #include <linux/seq_file.h>
 #include <linux/kernfs.h>
@@ -367,11 +366,11 @@ static inline void css_put_many(struct cgroup_subsys_state *css, unsigned int n)
  */
 #ifdef CONFIG_PROVE_RCU
 extern struct mutex cgroup_mutex;
-extern struct rw_semaphore css_set_rwsem;
+extern spinlock_t css_set_lock;
 #define task_css_set_check(task, __c)					\
 	rcu_dereference_check((task)->cgroups,				\
 		lockdep_is_held(&cgroup_mutex) ||			\
-		lockdep_is_held(&css_set_rwsem) ||			\
+		lockdep_is_held(&css_set_lock) ||			\
 		((task)->flags & PF_EXITING) || (__c))
 #else
 #define task_css_set_check(task, __c)					\