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BUG: KCSAN: data-race in enqueue_timer / timer_reduce

write to 0xffff88812345c518 of 8 bytes by task 26181 on cpu 0:
 hlist_add_head include/linux/list.h:787 [inline]
 enqueue_timer+0x6c/0x210 kernel/time/timer.c:540
 __internal_add_timer+0x4f/0x60 kernel/time/timer.c:553
 internal_add_timer kernel/time/timer.c:595 [inline]
 __mod_timer kernel/time/timer.c:1053 [inline]
 mod_timer kernel/time/timer.c:1100 [inline]
 add_timer+0x250/0x550 kernel/time/timer.c:1136
 __queue_delayed_work+0x13b/0x1d0 kernel/workqueue.c:1649
 queue_delayed_work_on+0xf3/0x110 kernel/workqueue.c:1674
 queue_delayed_work include/linux/workqueue.h:509 [inline]
 batadv_nc_start_timer net/batman-adv/network-coding.c:79 [inline]
 batadv_nc_worker+0x29d/0x390 net/batman-adv/network-coding.c:740
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
 worker_thread+0xa0/0x800 kernel/workqueue.c:2415
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff88812345c518 of 8 bytes by task 25781 on cpu 1:
 __mod_timer kernel/time/timer.c:963 [inline]
 timer_reduce+0x5d/0x830 kernel/time/timer.c:1115
 cgroup_file_notify+0x11f/0x130 kernel/cgroup/cgroup.c:4152
 pids_can_fork+0x16d/0x1b0 kernel/cgroup/pids.c:232
 cgroup_can_fork+0xde/0x1b0 kernel/cgroup/cgroup.c:5956
 copy_process+0x25bb/0x3b50 kernel/fork.c:2120
 _do_fork+0xfe/0x6e0 kernel/fork.c:2366
 __do_sys_clone kernel/fork.c:2521 [inline]
 __se_sys_clone kernel/fork.c:2502 [inline]
 __x64_sys_clone+0x12b/0x160 kernel/fork.c:2502
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 25781 Comm: syz-executor.5 Not tainted 5.4.0-rc6+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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