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BUG: KCSAN: data-race in del_timer / enqueue_timer

read to 0xffff8880a023e5b8 of 8 bytes by task 18613 on cpu 1:
 del_timer+0x3b/0xb0 kernel/time/timer.c:1198
 try_to_grab_pending+0x22d/0x3e0 kernel/workqueue.c:1246
 mod_delayed_work_on+0x50/0x100 kernel/workqueue.c:1703
 mod_delayed_work include/linux/workqueue.h:524 [inline]
 wb_queue_work+0x11a/0x290 fs/fs-writeback.c:188
 bdi_split_work_to_wbs+0x39d/0x6b0 fs/fs-writeback.c:876
 sync_inodes_sb+0x17a/0x530 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
 __do_sys_syncfs fs/sync.c:171 [inline]
 __se_sys_syncfs fs/sync.c:160 [inline]
 __x64_sys_syncfs+0x80/0xd0 fs/sync.c:160
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff8880a023e5b8 of 8 bytes by task 24396 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:1644
 queue_delayed_work_on+0xf3/0x110 kernel/workqueue.c:1669
 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:2264
 worker_thread+0xa0/0x800 kernel/workqueue.c:2410
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 24396 Comm: kworker/u4:4 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
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