[PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()

[Roman Gushchin <guro@fb.com>]

We've encountered a rcu stall in get_mem_cgroup_from_mm():

 rcu: INFO: rcu_sched self-detected stall on CPU
 rcu: 33-....: (21000 ticks this GP) idle=6c6/1/0x4000000000000002 softirq=35441/35441 fqs=5017
 (t=21031 jiffies g=324821 q=95837) NMI backtrace for cpu 33
 <...>
 RIP: 0010:get_mem_cgroup_from_mm+0x2f/0x90
 <...>
 __memcg_kmem_charge+0x55/0x140
 __alloc_pages_nodemask+0x267/0x320
 pipe_write+0x1ad/0x400
 new_sync_write+0x127/0x1c0
 __kernel_write+0x4f/0xf0
 dump_emit+0x91/0xc0
 writenote+0xa0/0xc0
 elf_core_dump+0x11af/0x1430
 do_coredump+0xc65/0xee0
 ? unix_stream_sendmsg+0x37d/0x3b0
 get_signal+0x132/0x7c0
 do_signal+0x36/0x640
 ? recalc_sigpending+0x17/0x50
 exit_to_usermode_loop+0x61/0xd0
 do_syscall_64+0xd4/0x100
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The problem is caused by an exiting task which is associated with
an offline memcg. We're iterating over and over in the
do {} while (!css_tryget_online()) loop, but obviously the memcg won't
become online and the exiting task won't be migrated to a live memcg.

Let's fix it by switching from css_tryget_online() to css_tryget().

As css_tryget_online() cannot guarantee that the memcg won't go
offline, the check is usually useless, except some rare cases
when for example it determines if something should be presented
to a user.

A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
css_tryget() instead of css_tryget_online() in task_get_css()").

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: stable@vger.kernel.org
Cc: Tejun Heo <tj@kernel.org>
---
 mm/memcontrol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 50f5bc55fcec..c5b5f74cfd4d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -939,7 +939,7 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
 			if (unlikely(!memcg))
 				memcg = root_mem_cgroup;
 		}
-	} while (!css_tryget_online(&memcg->css));
+	} while (!css_tryget(&memcg->css));
 	rcu_read_unlock();
 	return memcg;
 }
-- 
2.17.1