Messages in this thread | | | Date | Sat, 13 Jun 2015 19:45:27 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init_late() method |
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On 06/13, Ingo Molnar wrote: > > * Ingo Molnar <mingo@kernel.org> wrote: > > > * Oleg Nesterov <oleg@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.
Plus we need another mb() in sync_global_pgds(), say, before the main for_each_process() loop.
it would be nice to make this more simple/clear somehow...
Oleg.
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