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SubjectRe: [PATCH] mm: memcontrol: fix rcu unbalance in get_non_dying_memcg_end()
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On 4/28/26 5:59 PM, Shakeel Butt wrote:
> On Tue, Apr 28, 2026 at 11:06:21AM +0800, Qi Zheng 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>
>> ---
>> mm/memcontrol.c | 31 +++++++++++++++++++++----------
>> 1 file changed, 21 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index c3d98ab41f1f1..38f48a45b7ae5 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -805,12 +805,17 @@ static long memcg_state_val_in_pages(int idx, long val)
>> * Used in mod_memcg_state() and mod_memcg_lruvec_state() to avoid race with
>> * reparenting of non-hierarchical state_locals.
>> */
>> -static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *memcg)
>> +static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *memcg,
>> + bool *rcu_locked)
>> {
>> - if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
>> + /* Rebinding can cause this value to be changed at runtime */
>> + if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
>> + *rcu_locked = false;
>> return memcg;
>> + }
>>
>> rcu_read_lock();
>> + *rcu_locked = true;
>>
>> while (memcg_is_dying(memcg))
>> memcg = parent_mem_cgroup(memcg);
>> @@ -818,20 +823,23 @@ static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *me
>> return memcg;
>> }
>>
>> -static inline void get_non_dying_memcg_end(void)
>> +static inline void get_non_dying_memcg_end(bool rcu_locked)
>> {
>> - if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
>> + if (!rcu_locked)
>> return;
>>
>> rcu_read_unlock();
>> }
>> #else
>> -static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *memcg)
>> +static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *memcg,
>> + bool *rcu_locked)
>> {
>> + *rcu_locked = false;
>
> We don't need to set rcu_locked to false here as we don't access in !V1 build
> option.

Will do.

>
> With that fixed, you can add:
>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

Thanks!



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