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Does there exist a Kernel stack and a user-space stack for each user space process? If both stacks exist, there should be 2 stack pointers for each user space process right?
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There is one "kernel stack" per CPU like ISR Stack and one "kernel stack" per Process. There is one "user stack" for each process, though each thread has its own stack, including both user and kernel threads. Why kernel stack is used?
This means that each process requires its own stack and stack pointer. If process A gets suspended, the (single) thread of execution will switch to another process B and start using B's stack pointer. In this way, a single thread can execute all processes in a design unit without corrupting their stacks.
One process can have multiple threads, with each thread executing different code concurrently, while sharing data and synchronizing much more easily than cooperating processes. Threads require fewer system resources than processes, and can start more quickly.
A separate kernel stack is needed for each process to save the state of the process. The state needs to be saved in case a task switch is performed, i.e. the current process is put to sleep, and some other process scheduled to run.
In Linux, each task (userspace or kernel thread) has a kernel stack of either 8kb or 4kb, depending on kernel configuration. There are indeed separate stack pointers, however, only one is present in the CPU at any given time; if userspace code is running, the kernel stack pointer to be used on exceptions or interrupts is specified by the task-state segment, and if kernel code is running, the user stack pointer is saved in the context structure located on the kernel stack.
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