* Oleg Nesterov <o...@redhat.com> wrote:
> On 06/13, Ingo Molnar wrote:
> >
> > * Ingo Molnar <mi...@kernel.org> wrote:
> >
> > > * Oleg Nesterov <o...@redhat.com> wrote:
> > >
> > > >
> > > > 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.
>
> Hmm. that write must be visible before we start to _read_ *pgd_src,
> afaics.
>
> > > 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.
>
> I agree,
>
> > 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);
>
> Yes, this too.
>
> But wmb() can't help. At least we need the full mb() to serialize the
> STORE above (or list addition in copy_process) with the LOAD which
> reads *pgd_src.
True.
> Plus we need another mb() in sync_global_pgds(), say, before the main
> for_each_process() loop.
True.
So since we have a spin_lock() there already, I tried to find the right
primitive
to turn it into a full memory barrier but gave up. Is there a simple primitive
for
that?
Also, since this is x86 specific code we could rely on the fact that
spinlock-acquire is a full memory barrier?
>
> it would be nice to make this more simple/clear somehow...
I'm afraid lockless is rarely simple, but I'm open to suggestions ...
Thanks,
Ingo
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