Apache service is crashing on a Plesk server: AH00144: couldn’t grab the accept mutex

Symptoms

• Apache 2.4.10 crashes with the following error in its error log:

  (35)Resource deadlock avoided: AH00144: couldn't grab the accept mutex
  [fcgid:warn] [pid 14206] mod_fcgid: process 23611 graceful kill fail, sending SIGKILL
  [fcgid:warn] [pid 14206] mod_fcgid: process 22505 graceful kill fail, sending SIGKILL
  [core:alert] [pid 23133] AH00050: Child 22989 returned a Fatal error... Apache is exiting!

• Apache mpm_prefork mode is used:

  # apache2ctl -V | grep MPM
  Server MPM: prefork

Cause

Unclear.

Resolution

1. Connect to the Plesk server via SSH.

2. Open the file /etc/apache2/apache2.conf in a text editor. In this example, we are using the vi editor:

   # vi /etc/apache2/apache2.conf

3. Find the following line:

   Mutex file:${APACHE_LOCK_DIR} default

   and change it to:

   Mutex posixsem

4. Save the changes and close the file.

5. Restart the Apache service:

   # service apache2 restart

    

   If Apache continues to crash, switch Apache MPM mode to Event in Plesk at Tools & Settings > Apache Web Server.