I am working on an embedded Linux system (kernel-5.10.188), and use /dev/ttyS2 as serial console and ash in busybox is the login shell.
After logging in to system, I ran top -d 1 in the serial console (I am using mobaxterm in Windows 11 to acess the serial console), it worked well. Then I closed PC's lid (Windows suspended itself).
A few minutes later, I resumed the PC and found the serial connection in mobaxterm is down, I typed 'R' to re-connect the serial console, but there is NO output from the serial console.
I login to the system through adb shell, and I got followings.
top -d 1is process of PID, its status showed
Name: top
Umask: 0022
State: S (sleeping)
Tgid: 345
Ngid: 0
Pid: 345
PPid: 210
.....
voluntary_ctxt_switches: 51
nonvoluntary_ctxt_switches: 22
The last two lines showed the same value when cat /proc/345/status for several times. So the process is not running.
By running cat /proc/345/stack, it showed following.
# cat /proc/345/stack
[<0>] wait_woken+0x74/0x94
[<0>] n_tty_write+0x480/0x4f0
[<0>] file_tty_write.isra.36+0x1c8/0x358
[<0>] vfs_write+0x3e8/0x4d8
[<0>] ksys_write+0xe0/0x124
[<0>] syscall_common+0x34/0x58
The process is waiting in vfs_write and n_tty_write (I think it is from something printf or puts from top utility).
I can killed the top process with kill -9 345.
But there is still NO response in the console, so I checked the login shell process.
- Check the process of 210 (the login shell and the parent of
top).
# cat /proc/210/status
Name: sh
Umask: 0022
State: S (sleeping)
Tgid: 210
Ngid: 0
Pid: 210
PPid: 1
......
voluntary_ctxt_switches: 236
nonvoluntary_ctxt_switches: 45
# cat /proc/210/stack
[<0>] wait_woken+0x74/0x94
[<0>] n_tty_write+0x480/0x4f0
[<0>] file_tty_write.isra.36+0x1c8/0x358
[<0>] vfs_write+0x3e8/0x4d8
[<0>] ksys_write+0xe0/0x124
[<0>] syscall_common+0x34/0x58
The login shell is also in vfs_write and not being scheduled.
I have to kill -9 210 to bring back the login shell.
I can definely reproduce this issue with Windows suspend/resume. I went through the long list of kernel's commits on tty, but I did NOT find the same issue or the fix.
So what is the cause of this hang in serial console and how to fix it? Or where should I post this issue or bug for help?
