In the CPU-offline process, it calls mmdrop() after idle entry and the subsequent call to cpuhp_report_idle_dead(). Once execution passes the call to rcu_report_dead(), RCU is ignoring the CPU, which results in lockdep complaining when mmdrop() uses RCU from either memcg or debugobjects below. Fix it by cleaning up the active_mm state from BP instead. Every arch which has CONFIG_HOTPLUG_CPU should have already called idle_task_exit() from AP. The only exception is parisc because it switches them to &init_mm unconditionally (see smp_boot_one_cpu() and smp_cpu_init()), but the patch will still work there because it calls mmgrab(&init_mm) in smp_cpu_init() and then should call mmdrop(&init_mm) in finish_cpu(). WARNING: suspicious RCU usage ----------------------------- kernel/workqueue.c:710 RCU or wq_pool_mutex should be held! other info that might help us debug this: RCU used illegally from offline CPU! Call Trace: dump_stack+0xf4/0x164 (unreliable) lockdep_rcu_suspicious+0x140/0x164 get_work_pool+0x110/0x150 __queue_work+0x1bc/0xca0 queue_work_on+0x114/0x120 css_release+0x9c/0xc0 percpu_ref_put_many+0x204/0x230 free_pcp_prepare+0x264/0x570 free_unref_page+0x38/0xf0 __mmdrop+0x21c/0x2c0 idle_task_exit+0x170/0x1b0 pnv_smp_cpu_kill_self+0x38/0x2e0 cpu_die+0x48/0x64 arch_cpu_idle_dead+0x30/0x50 do_idle+0x2f4/0x470 cpu_startup_entry+0x38/0x40 start_secondary+0x7a8/0xa80 start_secondary_resume+0x10/0x14
Unified · Split
| @@ -167,7 +167,6 @@ static void pnv_smp_cpu_kill_self(void) · 166 unmodified lines | ||
| 167 | 167 | /* Standard hot unplug procedure */ |
| 168 | 168 | |
| 169 | 169 | idle_task_exit(); |
| 170 | − current->active_mm = NULL; /* for sanity */ | |
| 171 | 170 | cpu = smp_processor_id(); |
| 172 | 171 | DBG("CPU%d offline\n", cpu); |
| 173 | 172 | generic_set_cpu_dead(cpu); |
| @@ -49,6 +49,8 @@ static inline void mmdrop(struct mm_struct *mm) · 48 unmodified lines | ||
| 49 | 49 | __mmdrop(mm); |
| 50 | 50 | } |
| 51 | 51 | |
| 52 | +void mmdrop(struct mm_struct *mm); | |
| 53 | + | |
| 52 | 54 | /* |
| 53 | 55 | * This has to be called after a get_task_mm()/mmget_not_zero() |
| 54 | 56 | * followed by taking the mmap_sem for writing before modifying the |
| @@ -3,6 +3,7 @@ · 2 unmodified lines | ||
| 3 | 3 | * |
| 4 | 4 | * This code is licenced under the GPL. |
| 5 | 5 | */ |
| 6 | +#include <linux/sched/mm.h> | |
| 6 | 7 | #include <linux/proc_fs.h> |
| 7 | 8 | #include <linux/smp.h> |
| 8 | 9 | #include <linux/init.h> |
| @@ -564,6 +565,21 @@ static int bringup_cpu(unsigned int cpu) · 555 unmodified lines | ||
| 564 | 565 | return bringup_wait_for_ap(cpu); |
| 565 | 566 | } |
| 566 | 567 | |
| 568 | +static int finish_cpu(unsigned int cpu) | |
| 569 | +{ | |
| 570 | + struct task_struct *idle = idle_thread_get(cpu); | |
| 571 | + struct mm_struct *mm = idle->active_mm; | |
| 572 | + | |
| 573 | + /* | |
| 574 | + * idle_task_exit() will have switched to &init_mm, now | |
| 575 | + * clean up any remaining active_mm state. | |
| 576 | + */ | |
| 577 | + if (mm != &init_mm) | |
| 578 | + idle->active_mm = &init_mm; | |
| 579 | + mmdrop(mm); | |
| 580 | + return 0; | |
| 581 | +} | |
| 582 | + | |
| 567 | 583 | /* |
| 568 | 584 | * Hotplug state machine related functions |
| 569 | 585 | */ |
| @@ -1549,7 +1565,7 @@ static struct cpuhp_step cpuhp_hp_states[] = { · 979 unmodified lines | ||
| 1549 | 1565 | [CPUHP_BRINGUP_CPU] = { |
| 1550 | 1566 | .name = "cpu:bringup", |
| 1551 | 1567 | .startup.single = bringup_cpu, |
| 1552 | − .teardown.single = NULL, | |
| 1568 | + .teardown.single = finish_cpu, | |
| 1553 | 1569 | .cant_stop = true, |
| 1554 | 1570 | }, |
| 1555 | 1571 | /* Final state before CPU kills itself */ |
| @@ -6197,13 +6197,14 @@ void idle_task_exit(void) · 6196 unmodified lines | ||
| 6197 | 6197 | struct mm_struct *mm = current->active_mm; |
| 6198 | 6198 | |
| 6199 | 6199 | BUG_ON(cpu_online(smp_processor_id())); |
| 6200 | + BUG_ON(current != this_rq()->idle); | |
| 6200 | 6201 | |
| 6201 | 6202 | if (mm != &init_mm) { |
| 6202 | 6203 | switch_mm(mm, &init_mm, current); |
| 6203 | − current->active_mm = &init_mm; | |
| 6204 | 6204 | finish_arch_post_lock_switch(); |
| 6205 | 6205 | } |
| 6206 | − mmdrop(mm); | |
| 6206 | + | |
| 6207 | + /* finish_cpu(), as ran on the BP, will clean up the active_mm state */ | |
| 6207 | 6208 | } |
| 6208 | 6209 | |
| 6209 | 6210 | /* |