Messages in this thread |  | | Date | Tue, 28 Apr 2026 18:19:53 +0800 | | Subject | Re: [PATCH] mm: memcontrol: fix rcu unbalance in get_non_dying_memcg_end() | | From | Qi Zheng <> |
| |
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!
|  |