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BUG: KCSAN: data-race in __writeback_single_inode / iput

write to 0xffff8881017db5c8 of 8 bytes by task 8 on cpu 1:
 __writeback_single_inode+0x1e9/0x450 fs/fs-writeback.c:1503
 writeback_sb_inodes+0x564/0xf10 fs/fs-writeback.c:1732
 __writeback_inodes_wb fs/fs-writeback.c:1801 [inline]
 wb_writeback+0x327/0x700 fs/fs-writeback.c:1907
 wb_check_start_all fs/fs-writeback.c:2031 [inline]
 wb_do_writeback+0x330/0x510 fs/fs-writeback.c:2057
 wb_workfn+0xc8/0x520 fs/fs-writeback.c:2091
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2276
 worker_thread+0x636/0xae0 kernel/workqueue.c:2422
 kthread+0x1d0/0x1f0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff8881017db5c8 of 8 bytes by task 9641 on cpu 0:
 iput+0x2e/0x590 fs/inode.c:1676
 iterate_bdevs+0x192/0x2b0 fs/block_dev.c:1939
 ksys_sync+0x93/0xe0 fs/sync.c:118
 __do_sys_sync+0xa/0x10 fs/sync.c:125
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x0000000000020084 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 9641 Comm: syz-executor.0 Not tainted 5.13.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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