Re: Scheduler: SIGSTOP on multi threaded processes

["Richard B. Johnson" <linux-os@analogic.com>]

I don't think the kernel handler gets a chance to do anything
because SYS-V init installs its own handler(s). There are comments
about Linux misbehavior in the code. It turns out that I was
right about SIGSTOP and SIGCONT...


Source-code header..... Current init version is 2.85 but I can't find
the source. This is 2.62

/*
 * Init		A System-V Init Clone.
 *
 * Usage:	/sbin/init
 *		     init [0123456SsQqAaBbCc]
 *		  telinit [0123456SsQqAaBbCc]
 *
 * Version:	@(#)init.c  2.62  29-May-1996  MvS
 *
 *		This file is part of the sysvinit suite,

[SNIPPED...]

/*
 * Linux ignores all signals sent to init when the
 * SIG_DFL handler is installed. Therefore we must catch SIGTSTP
 * and SIGCONT, or else they won't work....
 *
 * The SIGCONT handler
 */
void cont_handler()
{
  got_cont = 1;
}

/*
 * The SIGSTOP & SIGTSTP handler
 */
void stop_handler()
{
  got_cont = 0;
  while(!got_cont) pause();
  got_cont = 0;
}


Now, if POSIX threads signals were implimented within the kernel,
without first purging the universe of all copies of the SYS-V init
that was distributed with early copies of RedHat and others (don't
know about current copies, a very long search failed to find the
source), then whatever you do in the kernel is wasted.

On Wed, 4 May 2005, Richard B. Johnson wrote:
> On Wed, 4 May 2005, Daniel Jacobowitz wrote:
>
> > On Wed, May 04, 2005 at 02:16:24PM -0400, Richard B. Johnson wrote:
> > > The kernel doesn't do SIGSTOP or SIGCONT. Within init, there is
> > > a SIGSTOP and SIGCONT handler. These can be inherited by others
> > > unless changed, perhaps by a 'C' runtime library. Basically,
> > > the SIGSTOP handler executes pause() until the SIGCONT signal
> > > is received.
> > >
> > > Any delay in stopping is the time necessary for the signal to
> > > be delivered. It is possible that the section of code that
> > > contains the STOP/CONT handler was paged out and needs to be
> > > paged in before the signal can be delivered.
> > >
> > > You might quicken this up by installing your own handler for
> > > SIGSTOP and SIGCONT....
> >
> > I don't know what RTOSes you've been working with recently, but none of
> > the above is true for Linux.  I don't think it ever has been.
> >
> > --
> > Daniel Jacobowitz
> > CodeSourcery, LLC
>
> Grab a copy of your favorite init source. SIGSTOP and SIGCONT are
> signals. They are handled by signal handlers, always have been
> on Unix and Unix clones like Linux.
>
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
> Notice : All mail here is now cached for review by Dictator Bush.
>                 98.36% of all statistics are fiction.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
 Notice : All mail here is now cached for review by Dictator Bush.
                 98.36% of all statistics are fiction.
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