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BUG: KCSAN: data-race in __mod_timer / expire_timers

write to 0xffff88813bd1bd88 of 8 bytes by interrupt on cpu 1:
 expire_timers+0x164/0x260 kernel/time/timer.c:1458
 __run_timers+0x338/0x3d0 kernel/time/timer.c:1731
 run_timer_softirq+0x19/0x30 kernel/time/timer.c:1744
 __do_softirq+0x13c/0x2c3 kernel/softirq.c:343
 run_ksoftirqd+0x13/0x20 kernel/softirq.c:650
 smpboot_thread_fn+0x34f/0x520 kernel/smpboot.c:165
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff88813bd1bd88 of 8 bytes by task 37 on cpu 0:
 __mod_timer+0x442/0xbf0 kernel/time/timer.c:1035
 add_timer+0x38/0x50 kernel/time/timer.c:1142
 __queue_delayed_work+0xec/0x150 kernel/workqueue.c:1655
 queue_delayed_work_on+0x78/0xc0 kernel/workqueue.c:1680
 queue_delayed_work include/linux/workqueue.h:522 [inline]
 batadv_forw_packet_queue+0x15e/0x1a0 net/batman-adv/send.c:696
 batadv_forw_packet_bcast_queue net/batman-adv/send.c:717 [inline]
 batadv_send_outstanding_bcast_packet+0x485/0x570 net/batman-adv/send.c:951
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x635/0xb90 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 37 Comm: kworker/u4:2 Not tainted 5.11.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: bat_events batadv_send_outstanding_bcast_packet
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