Re: [PATCH v2] sched/core: fix illegal RCU from offline CPUs

[Qian Cai]

> 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 donât 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)