Target a Specific Thread

Sometimes you just want to do some special actions for the thread you created, or just want to add a value to a thread. Pretty obvious you are going to add a field to the thread's metadata. In this section we will discuss just how to do that.

task_struct - A Thread's Metadata

In Linux Kernel, a thread is also known as a task. Process is a view from resource management, while threads, well, is a vew from thread management. Linux stores each thread's metadata inside a struct called task_struct. The definition of task_struct can be found in /include/linux/sched.h. As you can see, the whole struct contains tons of fields that are inside some #ifdef CONFIG_XXX_XXX, so this is also a warning to you that you should not define yours in them unless you know what you are doing.

Your Own Field in a Thread

It's not my job to teach you how to add a field to a struct in C language. This section only contains tips.

• After a few lines from the start of task_struct, you will see something like this. Follow the comment and add your field after this.

    /*
  	 * This begins the randomizable portion of task_struct. Only
  	 * scheduling-critical items should be added above here.
  	 */
  	randomized_struct_fields_start

• If you are just adding a generic field into the task_struct, be careful not to add your field into areas of a #ifdef CONFIG_XXX_XXX.

• Be sure that you are not making the task_struct too big.

The third tip is obvious. The first two tips could boil down to a simple and brainless rule: always add your fields directly after randomized_struct_fields_start.

Initializing a task_struct

Now you have your fields, you need to be responsible for them. You should have known that except for the very first thread, every thread is born from fork. So now things are clear. Two tasks: Initialize the first thread, deal with fork.

Initialize the very first thread

The very first task is called the init_task. You may find it in /init/init_task.c. It's pretty much a classic C struct variable initialization. Nothing too much to say, add your fields and it's done.

Note that in early versions of the kernel, init_task is located at /init/init_task.c, and is defined as a macro #define INIT_TASK(tsk).

Deal with fork

fork will clone one thread into two, so the task_struct is also copied. The behaviour of copying the task_struct happens in copy_process in /kernel/fork.c. copy_process is a really huge function that does tons of dirty works for us. Understanding it is not easy but we can quickly obeserve that several tens of lines later we find a lot of p->xxx = xxxx, which seems to be the place where the task_struct copying process happens. And indeed it is. Now you just add p->your_field = your_value and it's done. Do remember to be careful with #ifdef CONFIG_XXX_XXX.

If you want to inherit field value from the parent task_struct, you can do

    p->your_field = current->your_field;

For the meaning of current, read the following section.

current - Current task_struct

In a lot of time you want to access the task_struct of the current thread. Linux provides you with current identifier. To use it, include linux/sched.h. current is per-CPU defined so every thread on its own CPU will get its own task_struct.