On Ubuntu 18.04 server, apache crashed. On checking apache error log, found following
[Mon Aug 13 23:19:24.625927 2018] [mpm_prefork:emerg] [pid 2378] (43)Identifier removed: AH00144: couldn't grab the accept mutex [Mon Aug 13 23:19:24.626990 2018] [mpm_prefork:emerg] [pid 1227] (43)Identifier removed: AH00144: couldn't grab the accept mutex [Mon Aug 13 23:19:24.628515 2018] [mpm_prefork:emerg] [pid 1211] (43)Identifier removed: AH00144: couldn't grab the accept mutex [Mon Aug 13 23:19:24.628693 2018] [mpm_prefork:emerg] [pid 1309] (43)Identifier removed: AH00144: couldn't grab the accept mutex [Mon Aug 13 23:19:24.629122 2018] [mpm_prefork:emerg] [pid 2387] (43)Identifier removed: AH00144: couldn't grab the accept mutex [Mon Aug 13 23:19:24.629319 2018] [mpm_prefork:emerg] [pid 1603] (43)Identifier removed: AH00144: couldn't grab the accept mutex [Mon Aug 13 23:19:24.629483 2018] [mpm_prefork:emerg] [pid 1637] (43)Identifier removed: AH00144: couldn't grab the accept mutex [Mon Aug 13 23:19:24.629659 2018] [mpm_prefork:emerg] [pid 1566] (43)Identifier removed: AH00144: couldn't grab the accept mutex [Mon Aug 13 23:19:25.366503 2018] [core:alert] [pid 990] AH00050: Child 1211 returned a Fatal error... Apache is exiting! [Mon Aug 13 23:19:25.366568 2018] [:emerg] [pid 990] AH02818: MPM run failed, exiting
To fix the error, edit file
vi /etc/apache2/apache2.conf
Find
#Mutex file:${APACHE_LOCK_DIR} default
Replace with
Mutex posixsem
Restart Apache
service apache2 restart
See Apache