Messages in this thread | | | Date | Thu, 02 Feb 2012 16:57:58 +0200 | From | Avi Kivity <> | Subject | Re: [test result] dirty logging without srcu update -- Re: [RFC][PATCH] srcu: Implement call_srcu() |
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On 02/02/2012 04:44 PM, Takuya Yoshikawa wrote: > Avi Kivity <avi@redhat.com> wrote: > > > > I have one concern about correctness issue though: > > > > > > concurrent rmap write protection may not be safe due to > > > delayed tlb flush ... cannot happen? > > > > What do you mean by concurrent rmap write protection? > > > > Not sure, but other codes like: > > - mmu_sync_children() > for_each_sp(pages, sp, parents, i) > protected |= rmap_write_protect(vcpu->kvm, sp->gfn); > > if (protected) > kvm_flush_remote_tlbs(vcpu->kvm); > > - kvm_mmu_get_page() > if (rmap_write_protect(vcpu->kvm, gfn)) > kvm_flush_remote_tlbs(vcpu->kvm); > > I just wondered what can happen if GET_DIRTY_LOG is being processed > behind these processing?
It's a bug. If the flush happens outside the spinlock, then one of the callers can return before it is assured the tlb is flushed.
A B
spin_lock clear pte.w spin_unlock spin_lock pte.w already clear spin_unlock skip flush return flush
> > > They may find nothing to write protect and won't do kvm_flush_remote_tlbs() > if the gfn has been already protected by GET_DIRTY_LOG. > > But GET_DIRTY_LOG may still be busy write protecting other pages and > others can return before. (My code releases mmu_lock to not include > __put_user() in the critical section.) > > I am not still enough familier with these code yet.
It's actually an advantage, since you don't have any assumptions on how the code works.
> (maybe empty concern)
Nope, good catch of this subtle bug.
-- error compiling committee.c: too many arguments to function
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