Re: [uml-devel] Making UML Single Threader

[Blaisorblade <blaisorblade@yahoo.it>]

On Tuesday 08 November 2005 16:43, Jeff Dike wrote:
> On Mon, Nov 07, 2005 at 09:13:19PM -0800, Can Sar wrote:
> > Do the other threads (particularly the user thread) ever
> > do something else that would be important?
>
> The user thread doesn't.  The IO thread does, if you wish to do IO.

Since he needs to be _really_ single-threaded (and he may very possibly be 
trying to run something like Valgrind on UML) he may even run UBD without the 
I/O thread.

The code still exists, even if some bugs may have crept in since 2.6.9 (guess 
not, there has been no major rework). Basically, if io_thread_pid == -1, the 
code instead to to asynchronous I/O via this separate thread, does directly 
I/O by itself.

For normal usage, it's a bad idea - UML won't be able to schedule another 
thread until completion of that I/O request (much like green-threads do when 
implemented the simple way). Btw, usage of sigjmp()/longjmp() (between kernel 
threads) is actually green-threads said another way (like Jeff said below).

Btw, even userspace programs have a "stub" in the kernelspace thread.

In that case, through, it's just userspace(): an infinite loop running ptrace 
to intercept and nullify syscalls and segfaults, basically.

> And you can easily dispense with the sigio emulation thread.

> > Furthermore is the actual kernel process multi threaded?
>
> It does what amounts to user-level threads - i.e. there are multiple
> execution contexts in it, implemented with setjmp/longjmp, but that's
> not visible from the outside.

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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