The obj_cgroup_release() and memcg_reparent_objcgs() are serialized by
the css_set_lock. We do not need to care about objcg->memcg being
released in the process of obj_cgroup_release(). So there is no need
to pin memcg before releasing objcg. Remove those pinning logic to
simplfy the code.
There are only two places that modifies the objcg->memcg. One is the
initialization to objcg->memcg in the memcg_online_kmem(), another
is objcgs reparenting in the memcg_reparent_objcgs(). It is also
impossible for the two to run in parallel. So xchg() is unnecessary
and it is enough to use WRITE_ONCE().
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
mm/memcontrol.c | 20 +++++++-------------
1 file changed, 7 insertions(+), 13 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1f807448233e..90c1ac58c64c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -261,7 +261,6 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
static void obj_cgroup_release(struct percpu_ref *ref)
{
struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt);
- struct mem_cgroup *memcg;
unsigned int nr_bytes;
unsigned int nr_pages;
unsigned long flags;
@@ -291,11 +290,9 @@ static void obj_cgroup_release(struct percpu_ref *ref)
nr_pages = nr_bytes >> PAGE_SHIFT;
spin_lock_irqsave(&css_set_lock, flags);
- memcg = obj_cgroup_memcg(objcg);
if (nr_pages)
obj_cgroup_uncharge_pages(objcg, nr_pages);
list_del(&objcg->list);
- mem_cgroup_put(memcg);
spin_unlock_irqrestore(&css_set_lock, flags);
percpu_ref_exit(ref);
@@ -330,17 +327,14 @@ static void memcg_reparent_objcgs(struct mem_cgroup *memcg,
spin_lock_irq(&css_set_lock);
- /* Move active objcg to the parent's list */
- xchg(&objcg->memcg, parent);
- css_get(&parent->css);
- list_add(&objcg->list, &parent->objcg_list);
+ /* 1) Ready to reparent active objcg. */
+ list_add(&objcg->list, &memcg->objcg_list);
- /* Move already reparented objcgs to the parent's list */
- list_for_each_entry(iter, &memcg->objcg_list, list) {
- css_get(&parent->css);
- xchg(&iter->memcg, parent);
- css_put(&memcg->css);
- }
+ /* 2) Reparent active objcg and already reparented objcgs to parent. */
+ list_for_each_entry(iter, &memcg->objcg_list, list)
+ WRITE_ONCE(iter->memcg, parent);
+
+ /* 3) Move already reparented objcgs to the parent's list */
list_splice(&memcg->objcg_list, &parent->objcg_list);
spin_unlock_irq(&css_set_lock);
--
2.11.0
