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Message-Id: <C0F26F4C-77A0-41DF-856A-B7E29C56A4B6@lca.pw>
Date: Thu, 2 Apr 2020 10:00:16 -0400
From: Qian Cai <cai@....pw>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, juri.lelli@...hat.com,
dietmar.eggemann@....com, vincent.guittot@...aro.org,
rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
paulmck@...nel.org, tglx@...utronix.de,
"James.Bottomley@...senpartnership.com"
<James.Bottomley@...senPartnership.com>, deller@....de,
linuxppc-dev@...ts.ozlabs.org, linux-parisc@...r.kernel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2] sched/core: fix illegal RCU from offline CPUs
> On Apr 2, 2020, at 7:24 AM, Michael Ellerman <mpe@...erman.id.au> wrote:
>
> Qian Cai <cai@....pw> writes:
>> From: Peter Zijlstra <peterz@...radead.org>
>>
>> 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@lca.pw/
>
>> 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@....pw>
>> ---
>> 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@...erman.id.au> (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)
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