Re: [PATCH v2] mm: memcontrol: fix rcu unbalance in get_non_dying_memcg_end()

[Muchun Song <muchun.song@linux.dev>]

> On Apr 28, 2026, at 18:31, Qi Zheng <qi.zheng@linux.dev> wrote:
> 
> From: Qi Zheng <zhengqi.arch@bytedance.com>
> 
> 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.
> 
> 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>

Thanks.