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From: KAMEZAWA Hiroyuki on 10 May 2010 02:00
Hi, Sorry for my lack of study...maybe this is an acceptable one.
This patch fixes css_id's RCU check and revert a patch already merged.
This is based on linux-2.6.34-rc7.

-Kame
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com>

Commit ad4ba375373937817404fd92239ef4cadbded23b 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(a)jp.fujitsu.com>
---
kernel/cgroup.c | 15 +++++++++++++--
mm/memcontrol.c | 19 +++++--------------
2 files changed, 18 insertions(+), 16 deletions(-)

Index: linux-2.6.34-rc7/kernel/cgroup.c
===================================================================
--- linux-2.6.34-rc7.orig/kernel/cgroup.c
+++ linux-2.6.34-rc7/kernel/cgroup.c
@@ -4435,7 +4435,15 @@ __setup("cgroup_disable=", cgroup_disabl
*/
unsigned short css_id(struct cgroup_subsys_state *css)
{
- struct css_id *cssid = rcu_dereference(css->id);
+ struct css_id *cssid;
+
+ /*
+ * This css_id() can return correct value when somone has refcnt
+ * on this or this is under rcu_read_lock(). Once css->id is allocated,
+ * it's unchanged until freed.
+ */
+ cssid = rcu_dereference_check(css->id,
+ rcu_read_lock_held() || atomic_read(&css->refcnt));

if (cssid)
return cssid->id;
@@ -4445,7 +4453,10 @@ EXPORT_SYMBOL_GPL(css_id);

unsigned short css_depth(struct cgroup_subsys_state *css)
{
- struct css_id *cssid = rcu_dereference(css->id);
+ struct css_id *cssid;
+
+ cssid = rcu_dereference_check(css->id,
+ rcu_read_lock_held() || atomic_read(&css->refcnt));

if (cssid)
return cssid->depth;
Index: linux-2.6.34-rc7/mm/memcontrol.c
===================================================================
--- linux-2.6.34-rc7.orig/mm/memcontrol.c
+++ linux-2.6.34-rc7/mm/memcontrol.c
@@ -2314,9 +2314,7 @@ mem_cgroup_uncharge_swapcache(struct pag

/* 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(
{
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 v
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;
}

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