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BUG: KCSAN: data-race in sixpack_receive_buf / sixpack_receive_buf

write to 0xffff8880b493fa88 of 4 bytes by task 12433 on cpu 0:
 decode_data drivers/net/hamradio/6pack.c:837 [inline]
 decode_std_command drivers/net/hamradio/6pack.c:924 [inline]
 sixpack_decode drivers/net/hamradio/6pack.c:966 [inline]
 sixpack_receive_buf+0xa21/0xb90 drivers/net/hamradio/6pack.c:458
 tiocsti drivers/tty/tty_io.c:2197 [inline]
 tty_ioctl+0xb75/0xe10 drivers/tty/tty_io.c:2573
 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

read to 0xffff8880b493fa88 of 4 bytes by task 28 on cpu 1:
 decode_data drivers/net/hamradio/6pack.c:836 [inline]
 sixpack_decode drivers/net/hamradio/6pack.c:968 [inline]
 sixpack_receive_buf+0x3cc/0xb90 drivers/net/hamradio/6pack.c:458
 tty_ldisc_receive_buf+0xeb/0xf0 drivers/tty/tty_buffer.c:465
 tty_port_default_receive_buf+0x87/0xd0 drivers/tty/tty_port.c:38
 receive_buf drivers/tty/tty_buffer.c:481 [inline]
 flush_to_ldisc+0x1d5/0x260 drivers/tty/tty_buffer.c:533
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
 worker_thread+0xa0/0x800 kernel/workqueue.c:2415
 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: 1 PID: 28 Comm: kworker/u4:3 Not tainted 5.4.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events_unbound flush_to_ldisc
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