memcg: fix css_id() RCU locking for real

Commit ad4ba375373937817404fd92239ef4cadbded23b ("memcg: css_id() must be
called under rcu_read_lock()") modifies memcontol.c for fixing RCU check
message.  But Andrew Morton pointed out that the fix doesn't seems sane
and it was just for hidining lockdep messages.

This is a patch for do proper things.  Checking again, all places,
accessing without rcu_read_lock, that commit fixies was intentional....
all callers of css_id() has reference count on it.  So, it's not necessary
to be under rcu_read_lock().

Considering again, we can use rcu_dereference_check for css_id().  We know
css->id is valid if css->refcnt > 0.  (css->id never changes and freed
after css->refcnt going to be 0.)

This patch makes use of rcu_dereference_check() in css_id/depth and remove
unnecessary rcu-read-lock added by the commit.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 14 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0f711c213d2e..595d03f33b2c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2314,9 +2314,7 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
 
 	/* record memcg information */
 	if (do_swap_account && swapout && memcg) {
-		rcu_read_lock();
 		swap_cgroup_record(ent, css_id(&memcg->css));
-		rcu_read_unlock();
 		mem_cgroup_get(memcg);
 	}
 	if (swapout && memcg)
@@ -2373,10 +2371,8 @@ static int mem_cgroup_move_swap_account(swp_entry_t entry,
 {
 	unsigned short old_id, new_id;
 
-	rcu_read_lock();
 	old_id = css_id(&from->css);
 	new_id = css_id(&to->css);
-	rcu_read_unlock();
 
 	if (swap_cgroup_cmpxchg(entry, old_id, new_id) == old_id) {
 		mem_cgroup_swap_statistics(from, false);
@@ -4044,16 +4040,11 @@ static int is_target_pte_for_mc(struct vm_area_struct *vma,
 			put_page(page);
 	}
 	/* throught */
-	if (ent.val && do_swap_account && !ret) {
-		unsigned short id;
-		rcu_read_lock();
-		id = css_id(&mc.from->css);
-		rcu_read_unlock();
-		if (id == lookup_swap_cgroup(ent)) {
-			ret = MC_TARGET_SWAP;
-			if (target)
-				target->ent = ent;
-		}
+	if (ent.val && do_swap_account && !ret &&
+			css_id(&mc.from->css) == lookup_swap_cgroup(ent)) {
+		ret = MC_TARGET_SWAP;
+		if (target)
+			target->ent = ent;
 	}
 	return ret;
 }