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BUG: KCSAN: data-race in n_tty_read / n_tty_receive_buf_common

write to 0xffff88810aa1f9f4 of 4 bytes by task 8169 on cpu 0:
 tty_set_flow_change include/linux/tty.h:385 [inline]
 n_tty_check_unthrottle drivers/tty/n_tty.c:295 [inline]
 n_tty_read+0xaad/0x1220 drivers/tty/n_tty.c:2259
 tty_read+0xe6/0x200 drivers/tty/tty_io.c:869
 vfs_read+0x154/0x5c0 fs/read_write.c:494
 ksys_read+0xce/0x180 fs/read_write.c:634
 __do_sys_read fs/read_write.c:644 [inline]
 __se_sys_read fs/read_write.c:642 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:642
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff88810aa1f9f4 of 4 bytes by task 21 on cpu 1:
 __tty_set_flow_change include/linux/tty.h:380 [inline]
 n_tty_check_throttle drivers/tty/n_tty.c:272 [inline]
 n_tty_receive_buf_common+0x1db8/0x1df0 drivers/tty/n_tty.c:1761
 n_tty_receive_buf2+0x2e/0x40 drivers/tty/n_tty.c:1777
 tty_ldisc_receive_buf+0x61/0xf0 drivers/tty/tty_buffer.c:461
 tty_port_default_receive_buf+0x54/0x80 drivers/tty/tty_port.c:38
 receive_buf drivers/tty/tty_buffer.c:481 [inline]
 flush_to_ldisc+0x1c5/0x2b0 drivers/tty/tty_buffer.c:533
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 21 Comm: kworker/u4:1 Not tainted 5.10.0-rc2-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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