> On Apr 2, 2020, at 7:24 AM, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote: > > Qian Cai <cai@xxxxxx> writes: >> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> >> >> 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(). > > Thanks for debugging this. How did you hit it in the first place? Just repeatedly offline/online CPUs which will eventually cause an idle thread refcount goes to 0 and trigger __mmdrop() and of course it needs to enable lockdep (PROVE_RCU?) as well as having luck to hit the cgroup, workqueue or debugobject code paths to call RCU. > > A link to the original thread would have helped me: > > https://lore.kernel.org/lkml/20200113190331.12788-1-cai@xxxxxx/ > >> 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 > > Do we know when this started happening? ie. can we determine a Fixes > tag? I don’t know. I looked at some commits that it seems the code was like that even 10-year ago. It must be nobody who cares to run lockdep (PROVE_RCU?) with CPU hotplug very regularly. > >> <Peter to sign off here> >> Signed-off-by: Qian Cai <cai@xxxxxx> >> --- >> arch/powerpc/platforms/powernv/smp.c | 1 - >> include/linux/sched/mm.h | 2 ++ >> kernel/cpu.c | 18 +++++++++++++++++- >> kernel/sched/core.c | 5 +++-- >> 4 files changed, 22 insertions(+), 4 deletions(-) >> >> diff --git a/arch/powerpc/platforms/powernv/smp.c b/arch/powerpc/platforms/powernv/smp.c >> index 13e251699346..b2ba3e95bda7 100644 >> --- a/arch/powerpc/platforms/powernv/smp.c >> +++ b/arch/powerpc/platforms/powernv/smp.c >> @@ -167,7 +167,6 @@ static void pnv_smp_cpu_kill_self(void) >> /* Standard hot unplug procedure */ >> >> idle_task_exit(); >> - current->active_mm = NULL; /* for sanity */ > > If I'm reading it right, we'll now be running with active_mm == init_mm > in the offline loop. > > I guess that's fine, I can't think of any reason it would matter, and it > seems like we were NULL'ing it out just for paranoia's sake not because > of any actual problem. > > Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> (powerpc) > > > cheers > >> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h >> index c49257a3b510..a132d875d351 100644 >> --- a/include/linux/sched/mm.h >> +++ b/include/linux/sched/mm.h >> @@ -49,6 +49,8 @@ static inline void mmdrop(struct mm_struct *mm) >> __mmdrop(mm); >> } >> >> +void mmdrop(struct mm_struct *mm); >> + >> /* >> * This has to be called after a get_task_mm()/mmget_not_zero() >> * followed by taking the mmap_sem for writing before modifying the >> diff --git a/kernel/cpu.c b/kernel/cpu.c >> index 2371292f30b0..244d30544377 100644 >> --- a/kernel/cpu.c >> +++ b/kernel/cpu.c >> @@ -3,6 +3,7 @@ >> * >> * This code is licenced under the GPL. >> */ >> +#include <linux/sched/mm.h> >> #include <linux/proc_fs.h> >> #include <linux/smp.h> >> #include <linux/init.h> >> @@ -564,6 +565,21 @@ static int bringup_cpu(unsigned int cpu) >> return bringup_wait_for_ap(cpu); >> } >> >> +static int finish_cpu(unsigned int cpu) >> +{ >> + struct task_struct *idle = idle_thread_get(cpu); >> + struct mm_struct *mm = idle->active_mm; >> + >> + /* >> + * idle_task_exit() will have switched to &init_mm, now >> + * clean up any remaining active_mm state. >> + */ >> + if (mm != &init_mm) >> + idle->active_mm = &init_mm; >> + mmdrop(mm); >> + return 0; >> +} >> + >> /* >> * Hotplug state machine related functions >> */ >> @@ -1549,7 +1565,7 @@ static struct cpuhp_step cpuhp_hp_states[] = { >> [CPUHP_BRINGUP_CPU] = { >> .name = "cpu:bringup", >> .startup.single = bringup_cpu, >> - .teardown.single = NULL, >> + .teardown.single = finish_cpu, >> .cant_stop = true, >> }, >> /* Final state before CPU kills itself */ >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index a2694ba82874..8787958339d5 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -6200,13 +6200,14 @@ void idle_task_exit(void) >> struct mm_struct *mm = current->active_mm; >> >> BUG_ON(cpu_online(smp_processor_id())); >> + BUG_ON(current != this_rq()->idle); >> >> if (mm != &init_mm) { >> switch_mm(mm, &init_mm, current); >> - current->active_mm = &init_mm; >> finish_arch_post_lock_switch(); >> } >> - mmdrop(mm); >> + >> + /* finish_cpu(), as ran on the BP, will clean up the active_mm state */ >> } >> >> /* >> -- >> 2.21.0 (Apple Git-122.2)
