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Subject[PATCH v2] sched/core: fix illegal RCU from offline CPUs
Date
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 complaints when mmdrop() uses RCU from either memcg or
debugobjects, so it by scheduling mmdrop() on another online CPU.

According to the commit a79e53d85683 ("x86/mm: Fix pgd_lock deadlock"),
mmdrop() is not interrupt-safe, and called from
smp_call_function_single() could end up running mmdrop() from the IPI
interrupt handler.

Call Trace:
mmdrop+0x58/0x80
flush_smp_call_function_queue+0x128/0x2e0
smp_ipi_demux_relaxed+0x84/0xf0
doorbell_exception+0x118/0x588
h_doorbell_common+0x124/0x130
--- interrupt: e81 at replay_interrupt_return+0x0/0x4
LR = arch_local_irq_restore.part.8+0x78/0x90
arch_local_irq_restore.part.8+0x34/0x90 (unreliable)
_raw_spin_unlock_irq+0x50/0x80
finish_task_switch+0xd8/0x340
__schedule+0x4bc/0xba0
schedule_idle+0x38/0x70
do_idle+0x2a4/0x470
cpu_startup_entry+0x3c/0x40
start_secondary+0x7a8/0xa80
start_secondary_resume+0x10/0x14

Therefore, use mmdrop_async() instead to run mmdrop() from a process
context similar to the commit 7283094ec3db ("kernel, oom: fix potential
pgd_lock deadlock from __mmdrop").

=============================
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!
rcu_scheduler_active = 2, debug_locks = 1
2 locks held by swapper/37/0:
#0: c0000000010af608 (rcu_read_lock){....}, at:
percpu_ref_put_many+0x8/0x230
#1: c0000000010af608 (rcu_read_lock){....}, at:
__queue_work+0x7c/0xca0

stack backtrace:
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

=============================
WARNING: suspicious RCU usage
-----------------------------
kernel/sched/core.c:562 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

RCU used illegally from offline CPU!
rcu_scheduler_active = 2, debug_locks = 1
2 locks held by swapper/94/0:
#0: c000201cc77dc118 (&base->lock){-.-.}, at:
lock_timer_base+0x114/0x1f0
#1: c0000000010af608 (rcu_read_lock){....}, at:
get_nohz_timer_target+0x3c/0x2d0

stack backtrace:
Call Trace:
dump_stack+0xf4/0x164 (unreliable)
lockdep_rcu_suspicious+0x140/0x164
get_nohz_timer_target+0x248/0x2d0
add_timer+0x24c/0x470
__queue_delayed_work+0x8c/0x110
queue_delayed_work_on+0x128/0x130
__debug_check_no_obj_freed+0x2ec/0x320
free_pcp_prepare+0x1b4/0x570
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_prolog+0x10/0x14

Signed-off-by: Qian Cai <cai@lca.pw>
---

v2: use mmdrop_async() thanks to Tetsuo.

include/linux/sched/mm.h | 2 ++
kernel/fork.c | 2 +-
kernel/sched/core.c | 3 ++-
3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index c49257a3b510..205de134348c 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);
}

+extern void mmdrop_async(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/fork.c b/kernel/fork.c
index 080809560072..9a823a955b7c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -707,7 +707,7 @@ static void mmdrop_async_fn(struct work_struct *work)
__mmdrop(mm);
}

-static void mmdrop_async(struct mm_struct *mm)
+void mmdrop_async(struct mm_struct *mm)
{
if (unlikely(atomic_dec_and_test(&mm->mm_count))) {
INIT_WORK(&mm->async_put_work, mmdrop_async_fn);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 90e4b00ace89..1863a6fc4d82 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6194,7 +6194,8 @@ void idle_task_exit(void)
current->active_mm = &init_mm;
finish_arch_post_lock_switch();
}
- mmdrop(mm);
+ smp_call_function_single(cpumask_first(cpu_online_mask),
+ (void (*)(void *))mmdrop_async, mm, 0);
}

/*
--
2.21.0 (Apple Git-122.2)
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