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BUG: KCSAN: data-race in mm_update_next_owner / wb_workfn

read-write to 0xffff8880237cc02c of 4 bytes by task 10484 on cpu 0:
 wb_workfn+0x6d/0x410 fs/fs-writeback.c:2069
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x635/0xb90 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff8880237cc02c of 4 bytes by task 717 on cpu 1:
 mm_update_next_owner+0x1d9/0x4e0 kernel/exit.c:388
 exit_mm+0x327/0x420 kernel/exit.c:500
 do_exit+0x41f/0x1690 kernel/exit.c:812
 do_group_exit+0xc8/0x170 kernel/exit.c:922
 get_signal+0xd5e/0x1440 kernel/signal.c:2770
 arch_do_signal_or_restart+0x2a/0x270 arch/x86/kernel/signal.c:811
 handle_signal_work kernel/entry/common.c:147 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
 exit_to_user_mode_prepare+0x109/0x1a0 kernel/entry/common.c:201
 __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:302
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 717 Comm: syz-executor.2 Not tainted 5.11.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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