Re: + remove-the-likelypid-check-in-copy_process.patch added to -mm tree

[Oleg Nesterov <oleg@tv-sign.ru>]

On 03/16, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@tv-sign.ru> writes:
> 
> > Sukadev Bhattiprolu wrote:
> >
> > This means that idle threads (except "swapper") are visible to
> > for_each_process()
> > and do_each_thread(). Looks dangerous and somewhat strange to me.
> >
> > Could you explain this change?
> 
> Good catch.  I've been so busy pounding reviewing this patches into
> something that made sense that I missed the fact that we care about
> this for more than just the NULL pointer that would occur if we didn't
> do this.

Why it is bad to have a NULL pointer for idle thread? (Sorry for stupid
question, I can't track the code changes these days).

> Still it would be good if we could find a way to remove this rare
> special case.
> 
> Any chance we can undo what we don't want done for_idle, or create
> a factor of copy_process that only does as much as fork_idle should do,
> and make copy_process a wrapper that does the rest.
> 
> I doubt it is significant anywhere but it would be nice to remove a
> branch that except at boot up never happens.

... or at cpu-hotplug. Probably you are right, but I am not sure.

The "if (p->pid)" check in essence implements CLONE_UNHASHED flag,
it may be useful.

Btw. Looking at http://marc.theaimsgroup.com/?l=linux-mm-commits,

	 Subject: Explicitly set pgid and sid of init process
	 From: Sukadev Bhattiprolu <sukadev@us.ibm.com>

	 Explicitly set pgid and sid of init process to 1.

	 Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
	 Cc: Cedric Le Goater <clg@fr.ibm.com>
	 Cc: Dave Hansen <haveblue@us.ibm.com>
	 Cc: Serge Hallyn <serue@us.ibm.com>
	 Cc: Eric Biederman <ebiederm@xmission.com>
	 Cc: Herbert Poetzl <herbert@13thfloor.at>
	 Cc: <containers@lists.osdl.org>
	 Acked-by: Eric W. Biederman <ebiederm@xmission.com>
	 Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
	 ---

	  init/main.c |    1 +
	  1 file changed, 1 insertion(+)

	 diff -puN init/main.c~explicitly-set-pgid-and-sid-of-init-process init/main.c
	 --- a/init/main.c~explicitly-set-pgid-and-sid-of-init-process
	 +++ a/init/main.c
	 @@ -783,6 +783,7 @@ static int __init init(void * unused)
		  */
		 init_pid_ns.child_reaper = current;

	 +       __set_special_pids(1, 1);
		 cad_pid = task_pid(current);

		 smp_prepare_cpus(max_cpus);

Nice changelog :)

The patch looks good, except __set_special_pids(1, 1) should be no-op.
This is a child forked by swapper. copy_process() was changed by
	use-task_pgrp-task_session-in-copy_process.patch
, but signal->{pgrp,_session} get its value from INIT_SIGNALS ?

Could you explain this as well? Some other changes I missed?

Oleg.