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BUG: KCSAN: data-race in del_timer / enqueue_timer

write to 0xffff888120fc45b8 of 8 bytes by task 7771 on cpu 1:
 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]
 wb_wakeup_delayed+0xad/0xb0 mm/backing-dev.c:274
 wb_workfn+0x826/0x970 fs/fs-writeback.c:2095
 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 0xffff888120fc45b8 of 8 bytes by task 7623 on cpu 0:
 del_timer+0x3b/0xb0 kernel/time/timer.c:1198
 try_to_grab_pending+0x22d/0x3e0 kernel/workqueue.c:1251
 mod_delayed_work_on+0x50/0x100 kernel/workqueue.c:1708
 mod_delayed_work include/linux/workqueue.h:524 [inline]
 wb_queue_work+0x11a/0x290 fs/fs-writeback.c:188
 bdi_split_work_to_wbs+0x3c1/0x750 fs/fs-writeback.c:876
 sync_inodes_sb+0x179/0x410 fs/fs-writeback.c:2554
 __sync_filesystem fs/sync.c:34 [inline]
 sync_filesystem fs/sync.c:67 [inline]
 sync_filesystem+0x138/0x1a0 fs/sync.c:48
 ovl_sync_fs+0x92/0xa0 fs/overlayfs/super.c:276
 __sync_filesystem fs/sync.c:39 [inline]
 sync_filesystem fs/sync.c:67 [inline]
 sync_filesystem+0x168/0x1a0 fs/sync.c:48
 generic_shutdown_super+0x6b/0x260 fs/super.c:448
 kill_anon_super+0x2f/0x50 fs/super.c:1106
 deactivate_locked_super+0x64/0xb0 fs/super.c:335
 deactivate_super fs/super.c:366 [inline]
 deactivate_super+0xca/0xe0 fs/super.c:362
 cleanup_mnt+0x27f/0x330 fs/namespace.c:1102
 __cleanup_mnt+0x1f/0x30 fs/namespace.c:1109
 task_work_run+0xf6/0x130 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x2b4/0x2c0 arch/x86/entry/common.c:163
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x353/0x370 arch/x86/entry/common.c:300
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7623 Comm: syz-executor.2 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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