Re: [PATCH] exit: Clarify choice of new parent in forget_original_parent()

From: Oleg Nesterov
Date: Tue Jun 16 2015 - 16:05:08 EST

forgot to mention,

On 06/16, Oleg Nesterov wrote:
> On 06/16, Kirill Tkhai wrote:
> >
> > + * child_reaper doesn't have children after zap_pid_ns_processes(),
> > + * therefore it can't enter this function.
> > + */
> > + BUG_ON(child_reaper == father);
> Yes, we can add this BUG_ON(). But please see the comments in
> zap_pid_ns_processes(). We can change zap_pid_ns_processes() so that
> it returns with non-empty ->children list due to EXIT_DEAD children.
> Unlikely we will actually do this, at least soon, so I won't argue
> with this BUG_ON().
> But. In this case it would be better to add it into forget_original_parent(),
> reaper = find_new_reaper(...);
> BUG_ON(reaper == father);

because this way:

1. This BUG_ON() will still be valid even if we actually change
zap_pid_ns_processes() to return with EXIT_DEAD children

2. If we really want this sanity check, we should not tie it to
->child_reaper case.

OTOH. If for some reason you want to check ->child_reaper only, then
you should probably do this right after list_empty(&father->children)
check, or at least before find_alive_thread(). Because otherwise it
looks confusing, it looks as if "child_reaper == father" is only wrong
if find_alive_thread(father) fails.


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