Messages in this thread | | | Date | Sat, 13 Jun 2015 08:47:05 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init_late() method |
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* Oleg Nesterov <oleg@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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