Messages in this thread | | | Date | Sat, 13 Jun 2015 08:52:55 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init_late() method |
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* Ingo Molnar <mingo@kernel.org> wrote:
> * 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?
Ah, there's another detail I forgot. This might handle the fork case, but in exec() we have:
tsk->mm = mm; arch_pgd_init_late(mm);
and since the task is already linked, here we need the barrier.
So how about I improve the comment to:
/* * This function is called after a new MM has been made visible * in fork() or exec() via: * * tsk->mm = mm; * * This barrier makes sure the MM is visible to new RCU * walkers before we initialize the pagetables below, so that * we don't miss updates: */ smp_wmb();
and leave the barrier there?
Thanks,
Ingo
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