Comment 0 for bug 1565744

OS:
Ubuntu 14.04 LTS

Kernel:
3.13.0-79-generic x86_64

Apache:
2.4.7-1ubuntu4.5

In the default Apache 2.4 config on Ubuntu 14.04 LTS is the following set in /etc/apache2/apache2.conf:

Mutex file:${APACHE_LOCK_DIR} default

(/debian/config-dir/apache2.conf in http://archive.ubuntu.com/ubuntu/pool/main/a/apache2/apache2_2.4.7-1ubuntu4.5.debian.tar.gz)

which leads to the following output of "apache2ctl -t -D DUMP_RUN_CFG":

Mutex default: dir="/var/lock/apache2" mechanism=fcntl

This leads constantly to a lot of these warning/emergency messages on a server with 200 busy worker threads, 100 Requests/s, 300 KB/s:

[Tue Mar 08 16:08:18.596653 2016] [ssl:warn] [pid 8339:tid 140182179256064] (35)Resource deadlock avoided: AH02026: Failed to acquire SSL session cache lock

[Wed Mar 09 07:09:31.099331 2016] [mpm_worker:emerg] [pid 26526:tid 139668485949184] (35)Resource deadlock avoided: AH00273: apr_proc_mutex_lock failed. Attempting to shutdown process gracefully.

Solution (as suggested by Yann Ylavic from Apache):
Commenting (removing) the Mutex directive, which leads to the following output of "apache2ctl -t -D DUMP_RUN_CFG":

Mutex default: dir="/var/run/apache2/" mechanism=default

Then, there are no error messages anymore.

For the discussion, see the corresponding Apache httpd-users mailing list thread:

http://httpd.markmail.org/message/c7w5aujfmy2kfazi

(thread subject 'Lots of messages "[ssl:warn] Resource deadlock avoided: AH02026: Failed to acquire SSL session cache lock"' from 2016-03-08)

Here some more information:

# apache2ctl -V
Server version: Apache/2.4.7 (Ubuntu)
Server built: Jul 24 2015 17:25:11
Server's Module Magic Number: 20120211:27
Server loaded: APR 1.5.1-dev, APR-UTIL 1.5.3
Compiled using: APR 1.5.1-dev, APR-UTIL 1.5.3
Architecture: 64-bit
Server MPM: worker
  threaded: yes (fixed thread count)
    forked: yes (variable process count)
Server compiled with....
 -D APR_HAS_SENDFILE
 -D APR_HAS_MMAP
 -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
 -D APR_USE_SYSVSEM_SERIALIZE
 -D APR_USE_PTHREAD_SERIALIZE
 -D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
 -D APR_HAS_OTHER_CHILD
 -D AP_HAVE_RELIABLE_PIPED_LOGS
 -D DYNAMIC_MODULE_LIMIT=256
 -D HTTPD_ROOT="/etc/apache2"
 -D SUEXEC_BIN="/usr/lib/apache2/suexec"
 -D DEFAULT_PIDLOG="/var/run/apache2.pid"
 -D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
 -D DEFAULT_ERRORLOG="logs/error_log"
 -D AP_TYPES_CONFIG_FILE="mime.types"
 -D SERVER_CONFIG_FILE="apache2.conf"