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Linux Processes: Structure, Hangs and Core Dumps

Efficient and effective resolution practices

Now that the trace is running in window two, we need to issue the ll command in window one.

3.  Window one: Issue the ll command.

# ll test
rw-r--r--    1 chris    chris    46759 Sep    7 21:53 test

4.  Window two: Here are the results of the stdout and stopping the trace.

# strace -o /tmp/ll.strace -f -p 16935
Process 16935 attached <-- Trace running on 16935
Process 17424 attached <-- forked child process
Process 17424 detached <-- child ending returning to parent
Process 16935 detached <-- ctrl +c ending trace

The trace shows the fork() and execve() calls. Note that we are not showing the entire trace because so many system calls take place for each seemingly simple command.

...
16935 fork()       = 17424 <-- NEW task's PID
17424 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
17424 getpid()       = 17424
17424 rt_sigprocmask(SIG_SETMASK, [RTMIN], NULL, 8) = 0
17424 rt_sigaction(SIGTSTP, {SIG_DFL}, {SIG_IGN}, 8) = 0
17424 rt_sigaction(SIGTTIN, {SIG_DFL}, {SIG_IGN}, 8) = 0
17424 rt_sigaction(SIGTTOU, {SIG_DFL}, {SIG_IGN}, 8) = 0
17424 setpgid(17424, 17424)       = 0
17424 rt_sigprocmask(SIG_BLOCK, [CHLD TSTP TTIN TTOU], [RTMIN], 8) = 0
17424 ioctl(255, TIOCSPGRP, [17424]) = 0
17424 rt_sigprocmask(SIG_SETMASK, [RTMIN], NULL, 8) = 0
17424 rt_sigaction(SIGINT, {SIG_DFL}, {0x8087030, [], SA_RESTORER, \ 0x4005aca8}, 8) = 0
17424 rt_sigaction(SIGQUIT, {SIG_DFL}, {SIG_IGN}, 8) = 0
17424 rt_sigaction(SIGTERM, {SIG_DFL}, {SIG_IGN}, 8) = 0
17424 rt_sigaction(SIGCHLD, {SIG_DFL}, {0x80776a0, [], SA_RESTORER, \ 0x4005aca8}, 8) = 0
17424 execve("/bin/ls", ["ls", "-F", "--color=auto", "-l", "test"], \
[/* 56 vars */]) = 0

Summary of Process Creation
The fork() call creates a new task and assigns a PID, and this step is soon followed by the execve() call, executing the command along with its arguments. In this case, we see that the ll test command is actually ls -F --color=auto -l test.

Linux Process Termination
An understanding of process termination is useful for troubleshooting a process. As with process creation, the termination or exiting of a process is like that of any other UNIX flavor. If signal handling is implemented, the parent can be notified when its children terminate irregularly. Additionally, the parent process can also wait for the child to exit with some variation of wait(). When a process terminates or calls exit(), it returns its exit code to the caller (parent). At this point, the process is in a zombie or defunct state, waiting for the parent to reap the process. In some cases, the parent has long since died before the child. In these cases, the child has become orphaned, at which point init becomes the parent, and the return codes of the process are passed to init.

Linux Threads
No discussion of process fundamentals is complete without an explanation of Linux threads because an understanding of threads is crucial for troubleshooting processes. As mentioned earlier, the implementation of threads in Linux differs from that of UNIX because Linux threads are not contained within the proc structure. However, Linux does support multithreaded applications. "Multithreading" just means two or more threads working in parallel with each other while sharing the same address space. Multithreaded applications in Linux just use more than one task. Following this logic in the source, include/linux/sched.h shows that the task_struct structure maintains a one-to-one relationship with the task's thread through the use of a pointer to the thread_info structure, and this structure just points back to the task structure.

Excerpts from the source illustrate the one-to-one relationship between a Linux task and thread.
include/linux/sched.h

...
struct task_struct {
     volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
     struct thread_info *thread_info;
...

To see the thread_info structure point back to the task, we review include/asm-i386/thread_info.h.

...
    struct thread_info {
       struct task_struct *task; /* main task structure */
...


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James Kirkland is the advocate for Red Hat's initiatives and solutions for the Internet of Things(IoT) and is the architect of its three-tier strategy for IoT deployments. For the past five years, James has been focused on IoT solutions for the transportation and energy sectors. A frequent public speaker and writer on a wide range of technical topics, James is also the co-author of Linux Troubleshooting for System Administrators and Power Users (ISBN: 0131855158) published by Prentice Hall PTR. He has been working with UNIX and Linux variants over the course of 20 years in his positions at Red Hat, and in previous roles at Racemi and Hewlett-Packard.

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David Carmichael works for Hewlett-Packard as a technical problem manager in Alpharetta, Georgia. He earned a bachelors degree in computer science from West Virginia University in 1987 and has been helping customers resolve their IT problems ever since. David has written articles for HP's IT Resource Center (http://itrc.hp.com) and presented at HP World 2003.

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Greg Tinker began his career while at Bellsouth in Atlanta, Georgia. Greg joined Hewlett-Packard in 1999. Greg's primary role is as a storage business recovery specialist and has participated in HP World, taught several classes in Unix/Linux and Disk Array technology, and obtained various certifications including certifications in Advanced Clusters, SAN, and Linux.

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