Comment 11 for bug 1791758

I was able to verify/test in both Bionic and Cosmic proposed kernels, respectively: 4.15.0-44.47 and 4.18.0-14.15.

I don't have a reproducer, but to exercise the paths modified by the patches, the following approach was taken:

(a) Open ssh connection to the host/test machine, and run the following there:

DIR="/sys/kernel/debug/tracing"
echo tty_reopen > $DIR/set_ftrace_filter
echo function > $DIR/current_tracer

echo 'p:tty_name n_tty_receive_buf2 tty=+0x170(%di):string' > $DIR/kprobe_events
echo 1 > $DIR/events/kprobes/tty_name/enable

echo > trace

Then, start running the following loop:
$ while true; do pkill -9 -t pts/1; sleep 1; done

In this point, we don't have a pts/1 there, but keep it running.

(b) In another terminal from the ssh client, run:
$ while true; do ssh <host/test machine ip>; done

Notice it's interesting to have the following in the .ssh/config of the ssh client machine:
Host <test/host machine alias>
        ControlMaster auto
        ControlPath ~/.ssh/%r@%h-%p
        ControlPersist 600
in order to keep only one ssh connection opened.

(c) While the SSH in pts/1 is opened and killed automatically (and reopened by the loop), user must keep typing things in the keyboard in that terminal to force the tty flush.

(d) After running that for some seconds, one can verify in the trace output that the functions modified by the main patch in the SRUed series are there:

$ grep "pts1\|reopen" $DIR/trace|cut -f2- -d]|cut -f2- -d:|sort |uniq -c
     66 tty_name: (n_tty_receive_buf2+0x0/0x20) tty="pts1"
     60 tty_reopen <-tty_open

Also, the pattern showed in the trace file shows that the functions are called intermixed:
[...]
kworker/u56:1-3602 [000] .... 881.779225: tty_name: (n_tty_receive_buf2+0x0/0x20) tty="pts1"
kworker/u56:1-3602 [000] .... 881.861901: tty_name: (n_tty_receive_buf2+0x0/0x20) tty="pts1"
         sshd-3403 [023] .... 882.249355: tty_reopen <-tty_open
         bash-4052 [008] .... 882.250432: tty_reopen <-tty_open
         bash-4052 [008] .... 882.250441: tty_reopen <-tty_open
         bash-4052 [008] .... 882.251935: tty_reopen <-tty_open
kworker/u56:1-3602 [000] .... 882.440866: tty_name: (n_tty_receive_buf2+0x0/0x20) tty="pts1"
kworker/u56:1-3602 [000] .... 882.482994: tty_name: (n_tty_receive_buf2+0x0/0x20) tty="pts1"
[...]

Worth to notice that I've ran the test in 4.18.0-13 before, and I've noticed a small delay in the machine while running the test after updating to the -proposed version, probably due to the lock mechanism added.