[PATCH for-5.2-fixes] memcg: Don't loop on css_tryget_online() failure

[Tejun Heo <tj@kernel.org>]

A PF_EXITING task may stay associated with an offline css.
get_mem_cgroup_from_mm() may deadlock if mm->owner is in such state.
All similar logics in memcg are falling back to root memcg on
tryget_online failure and get_mem_cgroup_from_mm() can do the same.

A similar failure existed for task_get_css() and could be triggered
through BSD process accounting racing against memcg offlining.  See
18fa84a2db0e ("cgroup: Use css_tryget() instead of css_tryget_online()
in task_get_css()") for details.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 mm/memcontrol.c |   24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e50a2db5b4ff..be1fa89db198 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -918,23 +918,19 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
 
 	if (mem_cgroup_disabled())
 		return NULL;
+	/*
+	 * Page cache insertions can happen without an actual mm context,
+	 * e.g. during disk probing on boot, loopback IO, acct() writes.
+	 */
+	if (unlikely(!mm))
+		return root_mem_cgroup;
 
 	rcu_read_lock();
-	do {
-		/*
-		 * Page cache insertions can happen withou an
-		 * actual mm context, e.g. during disk probing
-		 * on boot, loopback IO, acct() writes etc.
-		 */
-		if (unlikely(!mm))
-			memcg = root_mem_cgroup;
-		else {
-			memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
-			if (unlikely(!memcg))
-				memcg = root_mem_cgroup;
-		}
-	} while (!css_tryget_online(&memcg->css));
+	memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
+	if (!css_tryget_online(&memcg->css))
+		memcg = root_mem_cgroup;
 	rcu_read_unlock();
+
 	return memcg;
 }
 EXPORT_SYMBOL(get_mem_cgroup_from_mm);