Re: distinguish kernel thread / user task

[Brian Gerst]

Sylvain wrote:
> Hi all,
>
> I have little question while doing some kernel implementation.
> How can I distinguish whether a task_struct is actually kernel thread
> or mere user task?
>
> My idea was to look at task_struct "mm" field to discriminate them,
> but that was wrong...
>
> Thanks,
>
> Sylvain

To the scheduler, a thread is a thread.  It doesn't care if it's a 
kernel thread or not.  The difference is execution context, which is 
cpu-dependant.  For example, on x86 the difference is in the code 
segment the task runs in.  Kernel threads run in KERNEL_CS (ring 0), and 
user threads run USER_CS (or any other ring 3 code segment, or vm86 mode 
set in eflags).  Other cpus might have a flag in the status register.

What are you trying to do that you need to know whether a thread is 
kernel or user?  I suppose if there were a compelling enough reason, a 
kernel/user flag could be added to the task struct, set in do_fork() for 
kernel threads, and cleared by execve().

--
				Brian Gerst
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