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BUG: KCSAN: data-race in pty_flush_buffer / pty_set_pktmode

write to 0xffff8880a408f1e4 of 11 bytes by task 9429 on cpu 1:
 pty_set_pktmode+0x116/0x130 drivers/tty/pty.c:191
 pty_unix98_ioctl+0x93/0x150 drivers/tty/pty.c:672
 tty_ioctl+0x93c/0xe10 drivers/tty/tty_io.c:2657
 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 0xffff8880a408f1e5 of 1 bytes by task 9439 on cpu 0:
 pty_flush_buffer+0x52/0xf0 drivers/tty/pty.c:232
 tty_driver_flush_buffer+0x49/0x60 drivers/tty/tty_ioctl.c:95
 mkiss_open+0x180/0x3eb drivers/net/hamradio/mkiss.c:715
 tty_ldisc_open.isra.0+0x6d/0xb0 drivers/tty/tty_ldisc.c:469
 tty_set_ldisc+0x224/0x3d0 drivers/tty/tty_ldisc.c:596
 tiocsetd drivers/tty/tty_io.c:2334 [inline]
 tty_ioctl+0x4c5/0xe10 drivers/tty/tty_io.c:2594
 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: 9439 Comm: syz-executor.3 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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