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Re: Can't End A Process

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Re: Can't End A Process

Subject: Re: Can't End A Process
From: Erik Buck <email@hidden>
Date: Mon, 15 Aug 2005 12:05:48 -0400

I have encountered unkillable processes on several flavors of Unix. When a Unix process dies, all file descriptors and heap memory are automatically released. [Beware of POSIX processor sets, message queues, shared memory, semaphores, and other resources that are not released automatically when a process dies or is killed.] On many Unix systems, any data buffered for a file descriptor is flushed when the file descriptor is closed/released. This includes file descriptors held open by mmap() and friends. This policy avoids inadvertent data loss.

The most common situation I see with unkillable processes is that some pending network IO such as a flush over NFS remains uncompleted and the process refuses to truly die until it completes. This is particularly problematic if the network connection used by NFS is down.

In the past, I have mitigated this problem by using unbuffered IO when possible even though that greatly degrades IO throughput. You can try explicitly closing all open file descriptors when the process receives a signal, but in my experience that just moves the same problem to a different location. Closing still hangs nearly forever, but at least it hangs from your code instead of somewhere deep in crt0 or the kernel.

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