Re: [uml-devel] Explaination of system call function flow in TT mode

[Blaisorblade <blaisorblade@yahoo.it>]

On Tuesday 17 May 2005 02:09, Young Koh wrote:
> Ok, now i'm getting close. Thanks a lot!!!
> but i have two more :)
>
> 1) in SKAS mode, the tracer (UML kernel) will execute a system call
> for the tracee (the user process). But what if the system call blocks?
> then, the tracer, the UML kernel, will be blocked, too, right? it
> means the whole UML system will be blocked?
Good question. However the tracee is one of the threads running inside UML, 
i.e. switch_to_skas switches between the different tracees. However, that's 
done only on timer interrupts, and possibly those interrupts are blocked by 
the syscall execution.

Indeed, I remember that when the UBD I/O is run by the UML kernel and not by a 
separate thread (this has been removed) the whole system becomes *really* 
slow, so probably the answer to your question is "Yes, but UML tries to avoid 
this".

> 2) in TT mode, what if the user process (tracee) installs its own
> signal handler for SIGUSR2?
The syscall is intercepted so it can't install signal handlers; the one it 
installs are handled inside the UML kernel.
-- 
Paolo Giarrusso, aka Blaisorblade
Skype user "PaoloGiarrusso"
Linux registered user n. 292729
http://www.user-mode-linux.org/~blaisorblade




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