CODEWITHSUNDEEP
cmultithreadingoperating-system
Bhupesh Pant
Bhupesh Pant

Reputation: 4349

Threads in User and kernel mode

what do we mean by thread running in User mode and running in kernel mode? Is this related to thread execution instruction from User mode and thread executing instruction from Kernel mode? Kindly elaborate. Also, is it possible that if a thread is executing in user mode is put to suspended state, then it may start executing in kernel mode? if yes, how is it possible? Until now I am only aware that a thread if suspended will be SUSPENDED completely, i.e. the context switch will take place by CPU to schedule another thread.

Upvotes: 4

Views: 4051

Answers (2)

David Schwartz
David Schwartz

Reputation: 182883

what do we mean by thread running in User mode and running in kernel mode?

There is no way to know what a person means by a phrase without context. If I had to guess, I'd say they are talking about whether the thread is scheduled by a user-space scheduler or a kernel scheduler. But it's also possible they are actually asking whether the thread is running user code or kernel code.

Is this related to thread execution instruction from User mode and thread executing instruction from Kernel mode? Kindly elaborate.

It could be. It also might not be. There's no way to know what a person means by a phrase without context.

Also, is it possible that if a thread is executing in user mode is put to suspended state, then it may start executing in kernel mode? if yes, how is it possible?

For implementations where the kernel schedules threads, the scheduler is running in kernel space. The code that actually suspends the thread typically runs in kernel space too because it has to add the thread to the various kernel scheduler data structures. So the thread that resumes the thread will run in kernel space too. At a higher level view, the same thread of execution can "become" the kernel scheduler, choose a user-space thread to execute, and then "become" that thread.

Until now I am only aware that a thread if suspended will be SUSPENDED completely, i.e. the context switch will take place by CPU to schedule another thread.

Right, and that's kernel code. So the same core is running user space code, then it's running kernel code, then it's running the user space code of another thread.

Upvotes: 2

Devolus
Devolus

Reputation: 22094

Modern operating systems have hardware support for separating the user code from the kernel code. On the x86 architecture you can set up memory pages that are not accessible to normal user code, and will trigger a page fault, so that the OS can survive faulty programs.

Code running in kernel mode has higher privileges, but also more responsibillities, as not everything is as easily accessible as from user space. If the user code gets stuck, then the OS can clean it up. If a kernel mode code hangs it might not be that easy, depending on how high the privilege level is.

Upvotes: 0

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