Linux User vs Kernel

从 stack 说起

函数调用链使用 stack 实现的。所谓内核也是由一个一个内核的函数实现的。所以，毫无疑问，内核的函数调用链也是用 stack 实现的。

同一个线程，两个执行栈

1. user stack -> 执行用户的代码，比如 main, 在 mm_struct 结构中 start_stack, 栈大小用 ulimit -s 查看
2. kernel stack -> 执行内核代码, 比如 syscall, 在 task_struct 结构中的 void *stack
3. user stack ===>context switch ===>kernel stack

两个栈怎样切换?

The same thread executes both user-space code and kernel code. It transitions between modes via system calls but remains a single logical thread managed by the kernel.

When a thread transitions between user and kernel mode:

1. execute code in user space:
   • When a thread executes code in user space (e.g., printf formatting), it runs in user mode with limited privileges. 这就是所谓的线程的用户态
2. CPU Automatically Switchs Stacks:
   • On system calls (e.g., write()), exceptions (e.g., page faults), or hardware interrupts, the CPU:
     • Saves the user-mode context (thread’s state: registers, instruction pointer, etc.) onto the kernel stack.
     • Switches to the kernel stack (predefined by the OS during thread creation).
3. Kernel Executes:
   • The kernel runs the requested operation (e.g., writing to disk, handling an interrupt). 这就是所谓的线程的内核态
4. Return to User Mode(非阻塞):
   • The kernel restores the user-mode context from the kernel stack and resumes execution in user space.
5. Blocking Behavior(阻塞):
   • If a system call blocks (e.g., waiting for disk I/O), the kernel marks the thread as TASK_INTERRUPTIBLE or TASK_UNINTERRUPTIBLE, and the scheduler runs other threads. When the I/O completes, the same thread resumes execution in user mode.

错误理解

• “Kernel Threads” vs. User Threads:
  • Some kernel-internal threads (kthreadd,ksoftirqd) exist purely in kernel space and handle background tasks. These are unrelated to user-space threads.
  • Your program’s threads are user-space threads that can enter kernel mode to request services but are not distinct from kernel threads. 用户的线程比如 main 线程可以进入 kernel mode，所谓内核态线程，但是线程实体是同一个。模式不同罢了。

write syscall 流程

1. 在用户空间执行代码，那就是用户态线程
2. 在内核空间执行代码，那就是内核态线程

User Space                Kernel Space
-----------              ---------------
[Thread A]               [Scheduler]
   ↓                         ↓
User mode:                Kernel mode:
printf() → write() → sys_write()
   ↓                         ↓
[User stack]           [Kernel stack]
   ↓                         ↓
Return to user code ←  Return from syscall

References

1. kernel stack
2. user stack