Memory controller use rcu_read_lock() in mem_cgroup_cache_charge()

Hugh Dickins noticed that we were using rcu_dereference() without rcu_read_lock() in the cache charging routine. The patch below fixes this problem

Signed-off-by Balbir Singh <[email protected]>

---

 mm/memcontrol.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff -puN mm/memcontrol.c~memory-controller-use-rcu-lead-lock mm/memcontrol.c

mm/memcontrol.c renamed from mm/memcontrol.c~memory-controller-use-rcu-lead-lock +7 -3

@@ -717,16 +717,20 @@ int mem_cgroup_charge(struct page *page,
 int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
 				gfp_t gfp_mask)
 {
+	int ret = 0;
 	struct mem_cgroup *mem;
 	if (!mm)
 		mm = &init_mm;
 
+	rcu_read_lock();
 	mem = rcu_dereference(mm->mem_cgroup);
+	css_get(&mem->css);
+	rcu_read_unlock();
 	if (mem->control_type == MEM_CGROUP_TYPE_ALL)
-		return mem_cgroup_charge_common(page, mm, gfp_mask,
+		ret = mem_cgroup_charge_common(page, mm, gfp_mask,
 				MEM_CGROUP_CHARGE_TYPE_CACHE);
-	else
-		return 0;
+	css_put(&mem->css);
+	return ret;
 }
 
 /*

_