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BUG: KCSAN: data-race in exit_signals / kthread_unpark

write to 0xffff888036b2d02c of 4 bytes by task 4260 on cpu 0:
 exit_signals+0x118/0x550 kernel/signal.c:2873
 do_exit+0x197/0x15e0 kernel/exit.c:781
 kthread+0x213/0x230 kernel/kthread.c:294
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff888036b2d02c of 4 bytes by task 4249 on cpu 1:
 to_kthread kernel/kthread.c:83 [inline]
 kthread_unpark+0x1f/0x150 kernel/kthread.c:536
 kthread_stop+0xa6/0x2c0 kernel/kthread.c:619
 vivid_stop_generating_vid_cap+0x757/0x780 drivers/media/test-drivers/vivid/vivid-kthread-cap.c:1007
 meta_cap_stop_streaming+0x4a/0x70 drivers/media/test-drivers/vivid/vivid-meta-cap.c:111
 __vb2_queue_cancel+0x72/0x6f0 drivers/media/common/videobuf2/videobuf2-core.c:1937
 vb2_core_streamoff drivers/media/common/videobuf2/videobuf2-core.c:2094 [inline]
 __vb2_cleanup_fileio drivers/media/common/videobuf2/videobuf2-core.c:2642 [inline]
 vb2_core_queue_release+0x37/0x1a0 drivers/media/common/videobuf2/videobuf2-core.c:2362
 vb2_queue_release drivers/media/common/videobuf2/videobuf2-v4l2.c:938 [inline]
 _vb2_fop_release drivers/media/common/videobuf2/videobuf2-v4l2.c:1111 [inline]
 vb2_fop_release+0xee/0x150 drivers/media/common/videobuf2/videobuf2-v4l2.c:1125
 vivid_fop_release+0x292/0x790 drivers/media/test-drivers/vivid/vivid-core.c:634
 v4l2_release+0xfa/0x1f0 drivers/media/v4l2-core/v4l2-dev.c:457
 __fput+0x263/0x4f0 fs/file_table.c:280
 ____fput+0x11/0x20 fs/file_table.c:313
 task_work_run+0x8e/0x110 kernel/task_work.c:140
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:174 [inline]
 exit_to_user_mode_prepare+0x17c/0x1b0 kernel/entry/common.c:208
 __syscall_exit_to_user_mode_work kernel/entry/common.c:290 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:301
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 4249 Comm: syz-executor.1 Tainted: G        W         5.12.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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