[CRIU] [PATCH] restore: define root_as_sibling before using it

[Tycho Andersen]

On Tue, Sep 09, 2014 at 11:23:47PM +0400, Pavel Emelyanov wrote:
> On 09/09/2014 10:15 PM, Tycho Andersen wrote:
> > root_as_sibling is used in criu_signals_setup(), but was only defined later
> > (when forking the root task for the first time). This meant that the
> > SA_NOCLDSTOP was never masked off, which meant SIGCHLD was never delivered
> > after ptracing the root task. Thus, when the a child of the root task died
> > (e.g. from cr_system), the root task sat in PTRACE_STOP, and the restore task
> > never PTRACE_CONT'd, resulting in a deadlock.
> > 
> > We also drop the pdeath_sig constraint from setting root_as_sibling when in
> > --restore-detached mode; in --restore-detached we /always/ need to have
> > root_as_sibling, but we only need to clone the parent if pdeath_sig is set and
> > we want to restore the task as alive.
> > 
> > v2: re-work the condition for CLONE_PARENT
> > 
> > Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
> > ---
> >  cr-restore.c | 30 ++++++++++++++++++++----------
> >  1 file changed, 20 insertions(+), 10 deletions(-)
> > 
> > diff --git a/cr-restore.c b/cr-restore.c
> > index 2735d0d..fe6c798 100644
> > --- a/cr-restore.c
> > +++ b/cr-restore.c
> > @@ -956,25 +956,21 @@ struct cr_clone_arg {
> >  static void maybe_clone_parent(struct pstree_item *item,
> >  			      struct cr_clone_arg *ca)
> >  {
> > +	/*
> > +	 * zdtm runs in kernel 3.11, which has the problem described below. We
> > +	 * avoid this by including the pdeath_sig test. Once users/zdtm migrate
> > +	 * off of 3.11, this condition can be simplified to just test
> > +	 * root_as_sibling.
> > +	 */
> >  	if (opts.swrk_restore ||
> >  	    (opts.restore_detach && ca->core->thread_core->pdeath_sig)) {
> >  		/*
> > -		 * This means we're called from lib's criu_restore_child().
> > -		 * In that case create the root task as the child one to+
> > -		 * the caller. This is the only way to correctly restore the
> > -		 * pdeath_sig of the root task. But also looks nice.
> > -		 *
> > -		 * Alternatively, if we are --restore-detached, a similar trick is
> > -		 * needed to correctly restore pdeath_sig and prevent processes from
> > -		 * dying once restored.
> > -		 *
> >  		 * There were a problem in kernel 3.11 -- CLONE_PARENT can't be
> >  		 * set together with CLONE_NEWPID, which has been solved in further
> >  		 * versions of the kernels, but we treat 3.11 as a base, so at
> >  		 * least warn a user about potential problems.
> >  		 */
> >  		item->rst->clone_flags |= CLONE_PARENT;
> > -		root_as_sibling = 1;
> >  		if (item->rst->clone_flags & CLONE_NEWPID)
> >  			pr_warn("Set CLONE_PARENT | CLONE_NEWPID but it might cause restore problem,"
> >  				"because not all kernels support such clone flags combinations!\n");
> > @@ -1792,6 +1788,20 @@ int cr_restore_tasks(void)
> >  {
> >  	int ret = -1;
> >  
> > +	if (opts.swrk_restore || opts.restore_detach) {
> > +		/*
> > +		 * This means we're called from lib's criu_restore_child().
> > +		 * In that case create the root task as the child one to+
> > +		 * the caller. This is the only way to correctly restore the
> > +		 * pdeath_sig of the root task. But also looks nice.
> > +		 *
> > +		 * Alternatively, if we are --restore-detached, a similar trick is
> > +		 * needed to correctly restore pdeath_sig and prevent processes from
> > +		 * dying once restored.
> > +		 */
> > +		root_as_sibling = 1;
> 
> The root_as_sibling and item->rst->clone_flags above should come
> in sync, they don't work one w/o another. Thus the whole complex
> logic about whether or not to do it should sit here and the
> maybe_clone_parent() should just check the root_as_sibling.

But isn't it wrong to --restore-detach and not set root_as_sibling?

Another problem is that we don't actually have the core when we are
setting up the signals, those are loaded in fork_with_pid.

Tycho

> > +	}
> > +
> >  	if (cr_plugin_init())
> >  		return -1;
> >  
> > 
>