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BUG: KCSAN: data-race in add_timer / run_timer_softirq

write to 0xffff88812c11dbc8 of 8 bytes by interrupt on cpu 1:
 expire_timers kernel/time/timer.c:1446 [inline]
 __run_timers kernel/time/timer.c:1774 [inline]
 __run_timers kernel/time/timer.c:1741 [inline]
 run_timer_softirq+0x440/0xbd0 kernel/time/timer.c:1787
 __do_softirq+0x118/0x34a kernel/softirq.c:292
 invoke_softirq kernel/softirq.c:373 [inline]
 irq_exit+0xb5/0xd0 kernel/softirq.c:413
 exiting_irq arch/x86/include/asm/apic.h:546 [inline]
 smp_apic_timer_interrupt+0xe2/0x270 arch/x86/kernel/apic/apic.c:1146
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
 native_safe_halt+0xe/0x10 arch/x86/include/asm/irqflags.h:60
 arch_safe_halt arch/x86/include/asm/paravirt.h:144 [inline]
 default_idle+0x21/0x170 arch/x86/kernel/process.c:695
 cpuidle_idle_call kernel/sched/idle.c:154 [inline]
 do_idle+0x1b7/0x290 kernel/sched/idle.c:269
 cpu_startup_entry+0x14/0x20 kernel/sched/idle.c:361
 start_secondary+0x164/0x1b0 arch/x86/kernel/smpboot.c:264
 secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:242

read to 0xffff88812c11dbc8 of 8 bytes by task 87 on cpu 0:
 __mod_timer kernel/time/timer.c:1027 [inline]
 add_timer+0x175/0x550 kernel/time/timer.c:1137
 __queue_delayed_work+0x13c/0x1d0 kernel/workqueue.c:1644
 queue_delayed_work_on+0xef/0x110 kernel/workqueue.c:1669
 queue_delayed_work include/linux/workqueue.h:509 [inline]
 batadv_mcast_start_timer net/batman-adv/multicast.c:71 [inline]
 batadv_mcast_mla_update+0xf10/0x1ae0 net/batman-adv/multicast.c:949
 process_one_work+0x424/0x930 kernel/workqueue.c:2264
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2410
 kthread+0x1cb/0x1f0 kernel/kthread.c:255
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 87 Comm: kworker/u4:3 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: bat_events batadv_mcast_mla_update
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