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Commit 7f0f1546 authored by KAMEZAWA Hiroyuki's avatar KAMEZAWA Hiroyuki Committed by Linus Torvalds
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memcg: fix css_id() RCU locking for real 



Commit ad4ba375 ("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: default avatarKAMEZAWA 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 11cad320

kernel/cgroup.c

+13 −2
Original line number Diff line number Diff line
@@ -4435,7 +4435,15 @@ __setup("cgroup_disable=", cgroup_disable); 
 */

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;

mm/memcontrol.c

+5 −14
Original line number Diff line number Diff line
@@ -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);
@@ -4044,17 +4040,12 @@ 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)) {

	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;

}