On Thu, Apr 02, 2020 at 10:00:16AM -0400, Qian Cai wrote: > > > > 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. I do run this combination quite frequently, but only as part of rcutorture, which might not be a representative workload. For one thing, it has a minimal userspace consisting only of a trivial init program. I don't recall having ever seen this. (I have seen one recent complaint about an IPI being sent to an offline CPU, but I cannot prove that this was not due to RCU bugs that I was chasing at the time.) Thanx, Paul > >> <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) >
