linux-kernel - Re: [test result] dirty logging without srcu update -- Re: [RFC][PATCH] srcu: Implement call

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Date:	Thu, 2 Feb 2012 23:44:50 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>,
	Peter Zijlstra <peterz@...radead.org>,
	paulmck@...ux.vnet.ibm.com, Oleg Nesterov <oleg@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [test result] dirty logging without srcu update -- Re:
 [RFC][PATCH] srcu: Implement call_srcu()

Avi Kivity <avi@...hat.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?

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.
(maybe empty concern)

	Takuya
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