KCSAN: data-race in echo_char / echo_char

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: b12d66a6 mm, kcsan: Instrument SLAB free with ASSERT_EXCLU..
git tree: https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=14d50931e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=10bc0131c4924ba9
dashboard link: https://syzkaller.appspot.com/bug?extid=7c7b586214244c8aa296
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [gre...@linuxfoundation.org jsl...@suse.com linux-...@vger.kernel.org el...@google.com]

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+7c7b58...@syzkaller.appspotmail.com

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

write to 0xffffc90003d8d018 of 8 bytes by task 16421 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:2200 [inline]
tty_ioctl+0xb75/0xe10 drivers/tty/tty_io.c:2576
vfs_ioctl fs/ioctl.c:47 [inline]
ksys_ioctl+0x109/0x150 fs/ioctl.c:763
__do_sys_ioctl fs/ioctl.c:772 [inline]
__se_sys_ioctl fs/ioctl.c:770 [inline]
__x64_sys_ioctl+0x4c/0x60 fs/ioctl.c:770
do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffc90003d8d018 of 8 bytes by task 2660 on cpu 1:
add_echo_byte drivers/tty/n_tty.c:845 [inline]
echo_char+0x13e/0x1c0 drivers/tty/n_tty.c:948
n_tty_receive_char_fast drivers/tty/n_tty.c:1448 [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+0x1739/0x1b00 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+0x413/0x8f0 kernel/workqueue.c:2264
worker_thread+0xa0/0x800 kernel/workqueue.c:2410
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: 2660 Comm: kworker/u4:4 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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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.