Re: [RFC][PATCH 1/3] Freezer: Fix vfork problem

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

On 02/28, Rafael J. Wysocki wrote:
>
> Okay, I have added a comment to freezer.h.  Please have a look.
>
>
> -extern void thaw_some_processes(int all);
> +/*
> + * The PF_FREEZER_SKIP flag should be set by a vfork parent right before it
> + * calls wait_for_completion(&vfork) and reset right after it returns from this
> + * function.  Next, the parent should call try_to_freeze() to freeze itself
> + * appropriately in case the child has exited before the freezing of tasks is
> + * complete.  However, we don't want kernel threads to be frozen in unexpected
> + * places, so we allow them to block freeze_processes() instead or to set
> + * PF_NOFREEZE if needed and PF_FREEZER_SKIP is only set for userland vfork
> + * parents.  Fortunately, in the ____call_usermodehelper() case the parent won't
> + * really block freeze_processes(), since ____call_usermodehelper() (the child)
> + * does a little before exec/exit and it can't be frozen before waking up the
> + * parent.
> + */

I think this comment is accurate and understandable, and I am not suggesting
to change it.

However, please note that PF_FREEZER_SKIP can be used not only for vfork().
For example, it seems to me we can also use freezer_...count() to solve the
problem with coredump. We can use the same "wait_for_completion_freezable"
pattern in exit_mm() and in coredump_wait(). (i do not claim this is a best
fix though).

Oleg.