mm: memcontrol: fix rcu unbalance in get_non_dying_memcg_end()

Currently, get_non_dying_memcg_start() and get_non_dying_memcg_end() both
evaluate cgroup_subsys_on_dfl(memory_cgrp_subsys) independently to
determine whether to acquire or release the RCU read lock.

However, the result of cgroup_subsys_on_dfl() can change dynamically at
runtime due to cgroup hierarchy rebinding (e.g., when the memory
controller is moved between cgroup v1 and v2 hierarchies).  This can cause
the following warning:

 =====================================
 WARNING: bad unlock balance detected!
 7.0.0-next-20260420+ #83 Tainted: G        W
 -------------------------------------
 memcg-repro/270 is trying to release lock (rcu_read_lock) at:
 [<ffffffff815f57f7>] rcu_read_unlock+0x17/0x60
 but there are no more locks to release!

 other info that might help us debug this:
 1 lock held by memcg-repro/270:
  #0: ffff888102fa2088 (vm_lock){++++}-{0:0}, at: do_user_addr_fault+0x285/0x880

 stack backtrace:
 CPU: 0 UID: 0 PID: 270 Comm: memcg-repro Tainted: G        W           7.0.0-next-20260420+ #
 Tainted: [W]=WARN
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
 Call Trace:
  <TASK>
  ? rcu_read_unlock+0x17/0x60
  dump_stack_lvl+0x77/0xb0
  print_unlock_imbalance_bug+0xe0/0xf0
  ? rcu_read_unlock+0x17/0x60
  lock_release+0x21d/0x2a0
  rcu_read_unlock+0x1c/0x60
  do_pte_missing+0x233/0xb40
  __handle_mm_fault+0x80e/0xcd0
  handle_mm_fault+0x146/0x310
  do_user_addr_fault+0x303/0x880
  exc_page_fault+0x9b/0x270
  asm_exc_page_fault+0x26/0x30
 RIP: 0033:0x5590e4eb41ea
 Code: 61 cc 66 0f 6f e0 66 0f 61 c2 66 0f db cd 66 0f 69 e2 66 0f 6f d0 66 0f 69 d4 66 0f 61 0
 RSP: 002b:00007ffcad25f030 EFLAGS: 00010202
 RAX: 00005590e4eb8010 RBX: 00007ffcad260f7d RCX: 00007f73c474d44d
 RDX: 00005590e4eb80a0 RSI: 00005590e4eb503c RDI: 000000000000000f
 RBP: 00005590e4eb70a0 R08: 0000000000000000 R09: 00007f73c483a680
 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
 R13: 00007ffcad25f180 R14: 00005590e4eb6dd8 R15: 00007f73c4869020
  </TASK>
 ------------[ cut here ]------------

Fix this by explicitly tracking the RCU lock state, ensuring that
rcu_read_unlock() in get_non_dying_memcg_end() is strictly paired with the
lock acquisition, regardless of any runtime rebinding events.

Link: https://lore.kernel.org/20260429073105.44472-1-qi.zheng@linux.dev
Fixes: 8285917d6f38 ("mm: memcontrol: prepare for reparenting non-hierarchical stats")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions