Re: [PATCH for 3.3] KVM: Fix write protection race during dirty logging

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 02/05/2012 07:42 PM, Takuya Yoshikawa wrote:

> From: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
> 
> This patch fixes a race introduced by:
> 
>   commit 95d4c16ce78cb6b7549a09159c409d52ddd18dae
>   KVM: Optimize dirty logging by rmap_write_protect()
> 
> During protecting pages for dirty logging, other threads may also try
> to protect a page in mmu_sync_children() or kvm_mmu_get_page().
> 
> In such a case, because get_dirty_log releases mmu_lock before flushing
> TLB's, the following race condition can happen:
> 
>   A (get_dirty_log)     B (another thread)
> 
>   lock(mmu_lock)
>   clear pte.w
>   unlock(mmu_lock)
>                         lock(mmu_lock)
>                         pte.w is already cleared
>                         unlock(mmu_lock)
>                         skip TLB flush
>                         return
>   ...
>   TLB flush
> 
> Though thread B assumes the page has already been protected when it
> returns, the remaining TLB entry will break that assumption.
> 
> This patch fixes this problem by making get_dirty_log hold the mmu_lock
> until it flushes the TLB's.
> 


I do not think this is a problem since the dirty page is logged when
the writeable spte is being set, and in the end of get_dirty_log, all
TLBs are always flushed.