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sched/core: Fix illegal RCU from offline CPUs

Browse source 
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().

  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

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Link: https://lkml.kernel.org/r/20200401214033.8448-1-cai@lca.pw
This commit is contained in:
Peter Zijlstra 2020-04-01 17:40:33 -04:00
parent f38f12d1e0
commit bf2c59fce4
4 changed files with 22 additions and 4 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

1
arch/powerpc/platforms/powernv/smp.c
View file

@ -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 */
cpu = smp_processor_id();
DBG("CPU%d offline\n", cpu);
generic_set_cpu_dead(cpu);

2
include/linux/sched/mm.h
View file

@ -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

18
kernel/cpu.c
View file

@ -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 */

5
kernel/sched/core.c
View file

@ -6197,13 +6197,14 @@ 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 */
}
/*