Understanding the linux kernel-ch3-Destroying Processes

Process Termination there are two system calls that terminate a User Mode application: · The exit_group( ) system call, which terminates a full thread group,  that is, a whole multithreaded application exit_group( ) do_group_exit( ) do_exit( ) · The _exit( ) system call, which terminates a single process,  regardless of any other process in the thread group of the victim _exit( )  do_exit( ) The do_group_exit( ) function 1,Checks whether the SIGNAL_GROUP_EXIT flag of the exiting process is not zero,  which means that the kernel already started an exit procedure for this thread group. In this case,  it considers as exit code the value stored in current->signal->group_exit_code, and jumps to step 4 2,Otherwise, it sets the SIGNAL_GROUP_EXIT flag of the process    and stores the termination code in the current->signal->group_exit_code field 3,Invokes the zap_other_threads( ) function to kill the other processes in the thread group of current,    if any. In order to do this, the function scans the per-PID list in the PIDTYPE_TGID hash table corresponding to current->tgid;     for each process in the list different from current, it sends a SIGKILL signal to it (see Chapter 11).     As a result, all such processes will eventually execute the do_exit( ) function, and thus they will be killed. 4,Invokes the do_exit( ) function passing to it the process termination code The do_exit( ) function 1. Sets the PF_EXITING flag in the flag field of the process descriptor to indicate that the process is being eliminated 2. Removes, if necessary, the process descriptor from a dynamic timer queue via the del_timer_sync( ) 3. Detaches from the process descriptor the data structures related to paging, semaphores,     filesystem, open file descriptors, namespaces, and I/O Permission Bitmap, respectively,      with the exit_mm( ), exit_sem( ), _ _exit_files( ), _ _exit_fs(), exit_namespace( ), and exit_thread( ) functions.     These functions also remove each of these data structures if no other processes are sharing them 4. If the kernel functions implementing the execution domain and the executable    format (see Chapter 20) of the process being killed are included in kernel modules, t    he function decreases their usage counters 5. Sets the exit_code field of the process descriptor to the process termination code.     This value is either the _exit( ) or exit_group( ) system call parameter    or an error code supplied by the kernel  6. Invokes the exit_notify( ) function  7. Invokes the schedule( ) function  Process Removal void release_task(struct task_struct * p) 1. Decreases the number of processes belonging to the user owner of the terminated process.  2. If the process is being traced, the function removes it from the debugger's ptrace_children list     and assigns the process back to its original parent 3. Invokes _ _exit_signal() to cancel any pending signal and to release the signal_struct descriptor of the process 4. Invokes _ _exit_sighand() to get rid of the signal handlers 5. Invokes _ _unhash_process( ), which in turn 6,signal to the parent of the leader to notify it of the death of the process 7. Invokes the sched_exit( ) function to adjust the timeslice of the parent process  8. Invokes put_task_struct() to decrease the process descriptor's usage counter;     if the counter becomes zero, the function drops any remaining reference to the process