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BUG: KCSAN: data-race in sixpack_receive_buf / sixpack_receive_buf

write to 0xffff888120a83ae6 of 1 bytes by task 21779 on cpu 0:
 decode_prio_command drivers/net/hamradio/6pack.c:875 [inline]
 sixpack_decode drivers/net/hamradio/6pack.c:964 [inline]
 sixpack_receive_buf+0x32b/0xba0 drivers/net/hamradio/6pack.c:458
 tiocsti drivers/tty/tty_io.c:2200 [inline]
 tty_ioctl+0xaf5/0xd80 drivers/tty/tty_io.c:2576
 vfs_ioctl fs/ioctl.c:47 [inline]
 ksys_ioctl+0x101/0x150 fs/ioctl.c:763
 __do_sys_ioctl fs/ioctl.c:772 [inline]
 __se_sys_ioctl fs/ioctl.c:770 [inline]
 __x64_sys_ioctl+0x47/0x60 fs/ioctl.c:770
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888120a83ae6 of 1 bytes by task 7193 on cpu 1:
 decode_prio_command drivers/net/hamradio/6pack.c:867 [inline]
 sixpack_decode drivers/net/hamradio/6pack.c:964 [inline]
 sixpack_receive_buf+0x2ce/0xba0 drivers/net/hamradio/6pack.c:458
 tty_ldisc_receive_buf+0xde/0xe0 drivers/tty/tty_buffer.c:465
 tty_port_default_receive_buf+0x82/0xd0 drivers/tty/tty_port.c:38
 receive_buf drivers/tty/tty_buffer.c:481 [inline]
 flush_to_ldisc+0x1dd/0x260 drivers/tty/tty_buffer.c:533
 process_one_work+0x424/0x930 kernel/workqueue.c:2264
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2410
 kthread+0x1cb/0x1f0 kernel/kthread.c:255
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7193 Comm: kworker/u4:2 Not tainted 5.6.0-rc1-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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