On 08/29/2013 05:31 PM, Gleb Natapov wrote:
> On Thu, Aug 29, 2013 at 02:50:51PM +0800, Xiao Guangrong wrote:
>> After more thinking, I still think rcu_assign_pointer() is unneeded when a entry
>> is removed. The remove-API does not care the order between unlink the entry and
>> the changes to its fields. It is the caller's responsibility:
>> - in the case of rcuhlist, the caller uses call_rcu()/synchronize_rcu(), etc to
>> enforce all lookups exit and the later change on that entry is invisible to the
>> lookups.
>>
>> - In the case of rculist_nulls, it seems refcounter is used to guarantee the order
>> (see the example from Documentation/RCU/rculist_nulls.txt).
>>
>> - In our case, we allow the lookup to see the deleted desc even if it is in slab cache
>> or its is initialized or it is re-added.
>>
> BTW is it a good idea? We can access deleted desc while it is allocated
> and initialized to zero by kmem_cache_zalloc(), are we sure we cannot
> see partially initialized desc->sptes[] entry? On related note what about
> 32 bit systems, they do not have atomic access to desc->sptes[].
Good eyes. This is a bug here.
It seems we do not have a good to fix this. How disable this optimization on
32 bit host, small changes:
static inline void kvm_mmu_rcu_free_page_begin(struct kvm *kvm)
{
+#ifdef CONFIG_X86_64
rcu_read_lock();
kvm->arch.rcu_free_shadow_page = true;
/* Set the indicator before access shadow page. */
smp_mb();
+#else
+ spin_lock(kvm->mmu_lock);
+#endif
}
static inline void kvm_mmu_rcu_free_page_end(struct kvm *kvm)
{
+#ifdef CONFIG_X86_64
/* Make sure that access shadow page has finished. */
smp_mb();
kvm->arch.rcu_free_shadow_page = false;
rcu_read_unlock();
+#else
+ spin_unlock(kvm->mmu_lock);
+#endif
}
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