It is sad, but eventually processes must die. When a process terminates, the kernel releases the resources owned by the process and notifies the child's parent of its unfortunate demise.
Typically, process destruction occurs when the process calls the exit() system call, either explicitly when it is ready to terminate or implicitly on return from the main subroutine of any program (that is, the C compiler places a call to exit() after main() returns). A process can also terminate involuntarily. This occurs when the process receives a signal or exception it cannot handle or ignore. Regardless of how a process terminates, the bulk of the work is handled by do_exit(), which completes a number of chores:
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First, it set the PF_EXITING flag in the flags member of the task_struct.
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Second, it calls del_timer_sync() to remove any kernel timers. Upon return, it is guaranteed that no timer is queued and that no timer handler is running.
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Next, if BSD process accounting is enabled, do_exit() call
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s acct_process() to write out accounting information.
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Now it calls __exit_mm() to release the mm_struct held by this process. If no other process is using this address space (in other words, if it is not shared), then deallocate it.
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Next, it calls exit_sem(). If the process is queued waiting for an IPC semaphore, it is dequeued here.
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It then calls __exit_files(), __exit_fs(), exit_namespace(), and exit_sighand() to decrement the usage count of objects related to file descriptors, filesystem data, the process namespace, and signal handlers, respectively. If any usage counts reach zero, the object is no longer in use by any process and it is removed.
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Subsequently, it sets the task's exit code, stored in the exit_code member of the task_struct, to the code provided by exit() or whatever kernel mechanism forced the termination. The exit code is stored here for optional retrieval by the parent.
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It then calls exit_notify() to send signals to the task's parent, reparents any of the task's children to another thread in their thread group or the init process, and sets the task's state to TASK_ZOMBIE.
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Finally, do_exit() calls schedule() to switch to a new process (see Chapter 4). Because TASK_ZOMBIE tasks are never scheduled, this is the last code the task will ever execute.
The code for do_exit() is defined in kernel/exit.c.
At this point, all objects associated with the task (assuming the task was the sole user) are freed. The task is not runnable (and in fact no longer has an address space in which to run) and is in the TASK_ZOMBIE state. The only memory it occupies is its kernel stack, the thread_info structure, and the task_struct structure. The task exists solely to provide information to its parent. After the parent retrieves the information, or notifies the kernel that it is uninterested, the remaining memory held by the process is freed and returned to the system for use.
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但是这段话让我困扰
second, it calls del_timer_sync() to remove any kernel timers. Upon return, it is guaranteed that no timer is queued and that no timer handler is running.
为什么是remove any kernel timers?
还有就是为什么要移除kernel timer ？
这里是谁和process相关的timer么？
还是说kernel会在管理这个process时需要用到一些timer？