open thought and learning

Semaphore problems on Apache

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I came across a simple but intriguing problem – apachectl restart will work and restart apache processes, and in my case restart the CA/Netegrity Siteminder agent as well. However, the server didn’t respond, and neither were there any messages on the error log. SM logs said the agent initialized successfully.

When I remove mod_sm.so and restart apache (after removing environment variables related to SM), everything worked just fine. I naturally assumed that the problem was with this module that I just removed.

It turned out that the problem was with this particular semaphore which didn’t release since about the last 24 hours, and was somehow linked to the siteminder agent module. After I did an ipcrm -s ID, everything was working fine as before.

I always thought semaphores/shared memory segments not freeing up will result in apache not restarting successfully. This is the first time apache didn’t complain on a restart, no logs displayed any errors, ‘removing’ a module rectified the error, and putting it back actually made the issue recur!

Need to learn more about semaphore allocation in linux.


Written by mohitsuley

August 16, 2008 at 2:08 am

Posted in linux, sysadmin

Tagged with , , ,

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