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BUG: KCSAN: data-race in add_timer / run_timer_softirq

write to 0xffff88812c01dbc8 of 8 bytes by interrupt on cpu 0:
 expire_timers kernel/time/timer.c:1451 [inline]
 __run_timers kernel/time/timer.c:1774 [inline]
 __run_timers kernel/time/timer.c:1741 [inline]
 run_timer_softirq+0xb1c/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
 __sanitizer_cov_trace_const_cmp4+0x4/0x20 kernel/kcov.c:274
 zap_pte_range mm/memory.c:1050 [inline]
 zap_pmd_range mm/memory.c:1184 [inline]
 zap_pud_range mm/memory.c:1213 [inline]
 zap_p4d_range mm/memory.c:1234 [inline]
 unmap_page_range+0x84e/0x1a60 mm/memory.c:1255
 unmap_single_vma+0x13c/0x1f0 mm/memory.c:1300
 unmap_vmas+0xe2/0x1b0 mm/memory.c:1332
 exit_mmap+0x13e/0x2f0 mm/mmap.c:3130
 __mmput kernel/fork.c:1088 [inline]
 mmput+0xe2/0x260 kernel/fork.c:1109
 exit_mm kernel/exit.c:485 [inline]
 do_exit+0x640/0x1880 kernel/exit.c:788
 do_group_exit+0xae/0x1a0 kernel/exit.c:899
 __do_sys_exit_group kernel/exit.c:910 [inline]
 __se_sys_exit_group kernel/exit.c:908 [inline]
 __x64_sys_exit_group+0x2b/0x30 kernel/exit.c:908
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812c01dbc8 of 8 bytes by task 43 on cpu 1:
 __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_purge_orig+0x46/0x50 net/batman-adv/originator.c:1380
 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: 1 PID: 43 Comm: kworker/u4:2 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_purge_orig
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