* Oleg Nesterov <o...@redhat.com> wrote:
> On 06/11, Ingo Molnar wrote:
> >
> > +void arch_pgd_init_late(struct mm_struct *mm, pgd_t *pgd)
> > +{
> > + /*
> > + * This is called after a new MM has been made visible
> > + * in fork() or exec().
> > + *
> > + * This barrier makes sure the MM is visible to new RCU
> > + * walkers before we initialize it, so that we don't miss
> > + * updates:
> > + */
> > + smp_wmb();
>
> I can't understand the comment and the barrier...
>
> Afaics, we need to ensure that:
>
> > + if (pgd_val(*pgd_src))
> > + WRITE_ONCE(*pgd_dst, *pgd_src);
>
> either we notice the recent update of this PGD, or (say) the subsequent
> sync_global_pgds() can miss the child.
>
> How the write barrier can help?
So the real thing this pairs with is the earlier:
tsk->mm = mm;
plus the linking of the new task in the task list.
_that_ write must become visible to others before we do the (conditional) copy
ourselves.
Granted, it happens quite a bit earlier, and the task linking's use of locking
is
a natural barrier - but since this is lockless I didn't want to leave a silent
assumption in.
Perhaps remove the barrier and just leave a comment in that describes the
assumption on task-linking being a full barrier?
Thanks,
Ingo
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