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

[Michael Ellerman]

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?

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?

> <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)