On 04/18/2013 07:00 PM, Gleb Natapov wrote:
> On Tue, Apr 16, 2013 at 02:32:46PM +0800, Xiao Guangrong wrote:
>> pte_list_clear_concurrently allows us to reset pte-desc entry
>> out of mmu-lock. We can reset spte out of mmu-lock if we can protect the
>> lifecycle of sp, we use this way to achieve the goal:
>>
>> unmap_memslot_rmap_nolock():
>> for-each-rmap-in-slot:
>> preempt_disable
>> kvm->arch.being_unmapped_rmap = rmapp
>> clear spte and reset rmap entry
>> kvm->arch.being_unmapped_rmap = NULL
>> preempt_enable
>>
>> Other patch like zap-sp and mmu-notify which are protected
>> by mmu-lock:
>> clear spte and reset rmap entry
>> retry:
>> if (kvm->arch.being_unmapped_rmap == rmap)
>> goto retry
>> (the wait is very rare and clear one rmap is very fast, it
>> is not bad even if wait is needed)
>>
> I do not understand what how this achieve the goal. Suppose that rmap
> == X and kvm->arch.being_unmapped_rmap == NULL so "goto retry" is skipped,
> but moment later unmap_memslot_rmap_nolock() does
> vm->arch.being_unmapped_rmap = X.
Access rmap is always safe since rmap and its entries are valid until
memslot is destroyed.
This algorithm protects spte since it can be freed in the protection of mmu-lock.
In your scenario:
======
CPU 1 CPU 2
vcpu / mmu-notify access the RMAP unmap rmap out of mmu-lock which is under
which is under mmu-lock slot-lock
zap spte1
clear RMAP entry
kvm->arch.being_unmapped_rmap = NULL,
do not wait
free spte1
set kvm->arch.being_unmapped_rmap = RMAP
walking RMAP and do not see spet1 on RMAP
(the entry of spte 1 has been reset by CPU 1)
set kvm->arch.being_unmapped_rmap = NULL
======
That protect CPU 2 can not access the freed-spte.
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