Scheduler: SIGSTOP on multi threaded processes

[Olivier Croquette <ocroquette@free.fr>]

Hello

On a 2.6.11 x86 system, I am SIGSTOP'ing processes which have started 
several threads before.

As expected, all threads are suspended.

But surprisingly, it can happen that some threads are still scheduled 
after the SIGSTOP has been issued.

Typically, they get scheduled 2 times within the next 5ms, before being 
really stopped.

Sadly, I could not reproduce that in a smaller example yet.

As this behaviour is IMA against the SIGSTOP concept, I tried to analyze 
the kernel code responsible for that. I could not really find the exact 
lines.

So here are my questions:

1. do you know any reason for which the SIGSTOP would not stop 
immediatly all threads of a process?

2. where do the threads get suspended exactly in the kernel? I think it 
is in signal.c but I am not sure exactly were.

3. can you confirm that the bug MUST be in my code? :)

Thanks!

Best regards

Olivier