How does the kernel use task_struct?

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I am a student doing research involving Linux processes, and I need to learn more about them to proceed with my experiment. In reading a couple books and things online, I've come across task_struct, which I'm not sure I'm understanding fully, and would like confirmation/correction onto my existing thought.

From what I think I've understood, task_struct is the C structure that acts as the process descriptor, holding everything the kernel might need to know about a processes. At the end of the process kernel stack lives another struct, thread_info, which has a pointer to the processes task_struct.

Another question: how does one access the task_struct of a process? Is there a calculation to find the location of it's thread_info? Is there a macro/function within the kernel?

kernel linux-kernel process linux c
2 Answers

Yes, the task_struct structure contains all the information about a process. You can obtain a pointer to the structure that describes the current process using the current macro as follows:

struct task_struct *p = current;

If you want to get the structure that describes a process given a pid, you can use the find_task_by_vpid function as follows:

read_lock(&tasklist_lock);
p = find_task_by_vpid(pid);
if (p) get_task_struct(p);
read_unlock(&tasklist_lock);
if (p == NULL) {
    // Task not found.
}

// Later, once you're finished with the task, execute:
put_task_struct(p);

Finally, if you want to iterate over all processes, you can use for_each_process as follows:

read_lock(&tasklist_lock);
for_each_process(p) {
    // p is a pointer to a task_struct instance.
}
read_unlock(&tasklist_lock);

If you want to an exclusive access to the task list to be able to make changes to one or more fields in the structure, write_lock_irqsave must be used instead of read_lock.

user_42306180

Let me try answer second question.

On the top of each process kernel stack there is a structure thread_info.

To get address of/pointer to thread_info, different architectures have different solutions, but on x86 it looks something like this:

To get pointer to thread_info just mask 13 bits of the stack pointer value:

movl $0xFFFFE000, %eax
andl %esp, %eax

Now we have pointer, and this structure has pointer for task_struct structure.

To get it, there is a current macro, which looks something like this:

#define current (current_thread_info()->task)

and it gives you pointer (address of) task_struct

user_95568320
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