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BUG: KCSAN: data-race in __mod_timer / timer_clear_idle

write to 0xffff88812c01cb24 of 1 bytes by task 0 on cpu 0:
 timer_clear_idle+0x2e/0x40 kernel/time/timer.c:1675
 tick_nohz_idle_retain_tick+0x3e/0x40 kernel/time/tick-sched.c:1004
 cpuidle_idle_call kernel/sched/idle.c:203 [inline]
 do_idle+0x290/0x320 kernel/sched/idle.c:269
 cpu_startup_entry+0x15/0x20 kernel/sched/idle.c:365
 rest_init+0xd8/0xe0 init/main.c:705
 arch_call_rest_init+0xa/0xb
 start_kernel+0x608/0x6d0 init/main.c:1043
 secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:243

read to 0xffff88812c01cb24 of 1 bytes by task 7 on cpu 1:
 forward_timer_base kernel/time/timer.c:891 [inline]
 __mod_timer+0xa24/0xf00 kernel/time/timer.c:1009
 add_timer+0x38/0x50 kernel/time/timer.c:1136
 __queue_delayed_work+0xea/0x150 kernel/workqueue.c:1649
 queue_delayed_work_on+0x8d/0xe0 kernel/workqueue.c:1674
 queue_delayed_work include/linux/workqueue.h:522 [inline]
 batadv_mcast_start_timer net/batman-adv/multicast.c:71 [inline]
 batadv_mcast_mla_update+0x18d9/0x1a10 net/batman-adv/multicast.c:949
 process_one_work+0x3cb/0x990 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x210/0x220 kernel/kthread.c:291
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7 Comm: kworker/u4:0 Not tainted 5.8.0-rc2-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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