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BUG: KCSAN: data-race in flush_workqueue / flush_workqueue

read to 0xffff88805e102050 of 8 bytes by task 13217 on cpu 1:
 flush_workqueue+0x30b/0xc00 kernel/workqueue.c:2834
 hci_dev_open+0x9e/0x180 net/bluetooth/hci_core.c:1604
 hci_sock_ioctl+0x2f8/0x600 net/bluetooth/hci_sock.c:1015
 sock_do_ioctl+0x61/0x230 net/socket.c:1038
 sock_ioctl+0x3a3/0x5e0 net/socket.c:1189
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:509 [inline]
 do_vfs_ioctl+0x991/0xc60 fs/ioctl.c:696
 ksys_ioctl+0xbd/0xe0 fs/ioctl.c:713
 __do_sys_ioctl fs/ioctl.c:720 [inline]
 __se_sys_ioctl fs/ioctl.c:718 [inline]
 __x64_sys_ioctl+0x4c/0x60 fs/ioctl.c:718
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff88805e102050 of 8 bytes by task 13213 on cpu 0:
 flush_workqueue+0x93f/0xc00 kernel/workqueue.c:2905
 hci_dev_open+0x9e/0x180 net/bluetooth/hci_core.c:1604
 hci_sock_ioctl+0x2f8/0x600 net/bluetooth/hci_sock.c:1015
 sock_do_ioctl+0x61/0x230 net/socket.c:1038
 sock_ioctl+0x3a3/0x5e0 net/socket.c:1189
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:509 [inline]
 do_vfs_ioctl+0x991/0xc60 fs/ioctl.c:696
 ksys_ioctl+0xbd/0xe0 fs/ioctl.c:713
 __do_sys_ioctl fs/ioctl.c:720 [inline]
 __se_sys_ioctl fs/ioctl.c:718 [inline]
 __x64_sys_ioctl+0x4c/0x60 fs/ioctl.c:718
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 13213 Comm: syz-executor.1 Not tainted 5.4.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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