When Apache on Ubuntu 18.04 is Forcefully Terminated

Since Apache suddenly dies, I checked the logs and found:

$ tail /var/log/apache2/error.log

[Mon Sep 03 19:24:17.252747 2018] [mpm_prefork:emerg] [pid 7851] (43)Identifier removed: AH00144: couldn't grab the accept mutex
[Mon Sep 03 19:24:17.253264 2018] [mpm_prefork:emerg] [pid 7819] (43)Identifier removed: AH00144: couldn't grab the accept mutex
[Mon Sep 03 19:24:17.253385 2018] [mpm_prefork:emerg] [pid 7815] (43)Identifier removed: AH00144: couldn't grab the accept mutex
[Mon Sep 03 19:24:17.253561 2018] [mpm_prefork:emerg] [pid 7817] (43)Identifier removed: AH00144: couldn't grab the accept mutex
[Mon Sep 03 19:24:17.253739 2018] [mpm_prefork:emerg] [pid 7852] (43)Identifier removed: AH00144: couldn't grab the accept mutex
[Mon Sep 03 19:24:17.253916 2018] [mpm_prefork:emerg] [pid 7855] (43)Identifier removed: AH00144: couldn't grab the accept mutex
[Mon Sep 03 19:24:17.252992 2018] [mpm_prefork:emerg] [pid 7848] (43)Identifier removed: AH00144: couldn't grab the accept mutex
[Mon Sep 03 19:24:17.464946 2018] [core:alert] [pid 7636] AH00050: Child 7815 returned a Fatal error... Apache is exiting!
[Mon Sep 03 19:24:17.464976 2018] [:emerg] [pid 7636] AH02818: MPM run failed, exitin

It looked like this.

Referencing "AH00144: couldn't grab the accept mutex in Apache 2.4 | gwaw.jp"
https://www.gwaw.jp/20180526-16.html

I did the following:

$ sudo bash -c "echo 'Mutex file:\${APACHE_LOCK_DIR} default' > /etc/apache2/conf-available/mutex-file.conf"

Then:

$ sudo a2enconf mutex-file
$ sudo systemctl restart apache2

And it worked.