KCSAN: data-race in echo_char / n_tty_receive_buf_common

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    05f22368 x86, kcsan: Enable KCSAN for x86
git tree:       https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=106bdb60e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
dashboard link: https://syzkaller.appspot.com/bug?extid=e518b0df8f4e19495d3e
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+e518b0df8f4e19495d3e@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KCSAN: data-race in echo_char / n_tty_receive_buf_common

write to 0xffffc9000c433018 of 8 bytes by task 18008 on cpu 0:
 add_echo_byte drivers/tty/n_tty.c:845 [inline]
 echo_char+0x14e/0x1c0 drivers/tty/n_tty.c:948
 n_tty_receive_char_special+0xb5f/0x1c10 drivers/tty/n_tty.c:1339
 n_tty_receive_buf_fast drivers/tty/n_tty.c:1610 [inline]
 __receive_buf drivers/tty/n_tty.c:1644 [inline]
 n_tty_receive_buf_common+0x1844/0x1b00 drivers/tty/n_tty.c:1742
 n_tty_receive_buf+0x3a/0x50 drivers/tty/n_tty.c:1771
 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 0xffffc9000c433018 of 8 bytes by task 7 on cpu 1:
 flush_echoes drivers/tty/n_tty.c:828 [inline]
 __receive_buf drivers/tty/n_tty.c:1648 [inline]
 n_tty_receive_buf_common+0xe3f/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
 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: 7 Comm: kworker/u4:0 Not tainted 5.4.0-rc3+ #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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