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Subject: Re: INFO: rcu detected stall in sys_keyctl
From:   Kris Karas <linux-1993@moonlit-rail.com>
To:     LKML <linux-kernel@vger.kernel.org>
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Dmitry Vyukov wrote:
> syzbot wrote:
>> Call Trace:
>>   <IRQ>
>>   __dump_stack lib/dump_stack.c:77 [inline]
>>   dump_stack+0x197/0x210 lib/dump_stack.c:118
>>   nmi_cpu_backtrace.cold+0x70/0xb2 lib/nmi_backtrace.c:101
>>   nmi_trigger_cpumask_backtrace+0x23b/0x28b lib/nmi_backtrace.c:62
>>   arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
>>   trigger_single_cpu_backtrace include/linux/nmi.h:164 [inline]
>>   rcu_dump_cpu_stacks+0x183/0x1cf kernel/rcu/tree_stall.h:254
>>   print_cpu_stall kernel/rcu/tree_stall.h:475 [inline]
>>   check_cpu_stall kernel/rcu/tree_stall.h:549 [inline]
>>   rcu_pending kernel/rcu/tree.c:3030 [inline]
>>   rcu_sched_clock_irq.cold+0x51a/0xc37 kernel/rcu/tree.c:2276
>>   update_process_times+0x2d/0x70 kernel/time/timer.c:1726
>>   tick_sched_handle+0xa2/0x190 kernel/time/tick-sched.c:171
>>   tick_sched_timer+0x53/0x140 kernel/time/tick-sched.c:1314
>>   __run_hrtimer kernel/time/hrtimer.c:1517 [inline]
>>   __hrtimer_run_queues+0x364/0xe40 kernel/time/hrtimer.c:1579
>>   hrtimer_interrupt+0x314/0x770 kernel/time/hrtimer.c:1641
>>   local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1119 [inline]
>>   smp_apic_timer_interrupt+0x160/0x610 arch/x86/kernel/apic/apic.c:1144
>>   apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
>>   </IRQ>
>>
> +lib/mpi maintainers
>
> I wonder if this can also be triggered by remote actors (tls, wifi, usb, etc).
>

This looks somewhat similar to an OOPS + rcu stall I reported earlier in 
reply to Greg KH's announcement of 5.5.7:

	rcu: INFO: rcu_sched self-detected stall on CPU
	rcu:    14-....: (20999 ticks this GP) idle=216/1/0x4000000000000002 softirq=454/454 fqs=5250
	        (t=21004 jiffies g=-755 q=1327)
	NMI backtrace for cpu 14
	CPU: 14 PID: 520 Comm: pidof Tainted: G      D           5.5.7 #1
	Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./X470 Taichi, BIOS P3.50 07/18/2019
	Call Trace:
	 <IRQ>
	 dump_stack+0x50/0x70
	 nmi_cpu_backtrace.cold+0x14/0x53
	 ? lapic_can_unplug_cpu.cold+0x44/0x44
	 nmi_trigger_cpumask_backtrace+0x7b/0x88
	 rcu_dump_cpu_stacks+0x7b/0xa9
	 rcu_sched_clock_irq.cold+0x152/0x39b
	 update_process_times+0x1f/0x50
	 tick_sched_timer+0x40/0x90
	 ? tick_sched_do_timer+0x50/0x50
	 __hrtimer_run_queues+0xdd/0x180
	 hrtimer_interrupt+0x108/0x230
	 smp_apic_timer_interrupt+0x53/0xa0
	 apic_timer_interrupt+0xf/0x20
	 </IRQ>

I don't have a reproducer for it, either.  It showed up in 5.5.7 (but 
might be from earlier as it reproduces so infrequently).

Kris