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BUG: KCSAN: data-race in commit_echoes / process_echoes

read to 0xffffc90001791028 of 8 bytes by task 11400 on cpu 1:
 process_echoes+0x3f/0x100 drivers/tty/n_tty.c:806
 n_tty_write+0x132/0x9f0 drivers/tty/n_tty.c:2319
 do_tty_write drivers/tty/tty_io.c:962 [inline]
 tty_write+0x2fb/0x4e0 drivers/tty/tty_io.c:1046
 __vfs_write+0x67/0xc0 fs/read_write.c:494
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffffc90001791028 of 8 bytes by task 20735 on cpu 0:
 commit_echoes+0x5c/0x170 drivers/tty/n_tty.c:780
 n_tty_receive_char_fast drivers/tty/n_tty.c:1449 [inline]
 n_tty_receive_buf_fast drivers/tty/n_tty.c:1609 [inline]
 __receive_buf drivers/tty/n_tty.c:1644 [inline]
 n_tty_receive_buf_common+0x1741/0x1b00 drivers/tty/n_tty.c:1742
 n_tty_receive_buf2+0x3d/0x60 drivers/tty/n_tty.c:1777
 tty_ldisc_receive_buf+0x71/0xf0 drivers/tty/tty_buffer.c:461
 paste_selection+0x15a/0x270 drivers/tty/vt/selection.c:372
 tioclinux+0x104/0x3e0 drivers/tty/vt/vt.c:3044
 vt_ioctl+0x1523/0x1c80 drivers/tty/vt/vt_ioctl.c:364
 tty_ioctl+0x93c/0xe10 drivers/tty/tty_io.c:2660
 vfs_ioctl fs/ioctl.c:47 [inline]
 file_ioctl fs/ioctl.c:545 [inline]
 do_vfs_ioctl+0x84f/0xcf0 fs/ioctl.c:732
 ksys_ioctl+0xbd/0xe0 fs/ioctl.c:749
 __do_sys_ioctl fs/ioctl.c:756 [inline]
 __se_sys_ioctl fs/ioctl.c:754 [inline]
 __x64_sys_ioctl+0x4c/0x60 fs/ioctl.c:754
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 20735 Comm: syz-executor.4 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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