Re: [uml-devel] How is Preemption implemented in User Mode Linux?

[Rob Landley <rob@landley.net>]

On 05/26/2013 03:08:17 AM, richard -rw- weinberger wrote:
> On Sun, May 26, 2013 at 1:01 AM, Hopsing K <hopsingk@gmail.com> wrote:
> > Some question at the side: can you call longjmp from within a signal
> > handler? My first assumption was that UML uses longjmp from within
> > the SIGALRM handler to implement preemption, butseems not to .

Yes, "man longjmp" shows you sigsetjmp() and siglongjmp(), which save  
and restore the signal mask. This is an important detail because the  
signal you're handling is disabled in the signal handler (to avoid an  
infinite loop). You can just setjmp() and longjmp() and fiddle with the  
signal mask yourself, but signals can nest (a different signal can  
interrupt a signal handler) so re-enabling just one may not be enough.

On return, the kernel re-enables the signal you were handling, even  
from nested signal handlers. But if you longjmp() you don't call the  
kernel return code...

Rob
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