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

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Date:	Thu, 02 Feb 2012 16:57:58 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Takuya Yoshikawa <takuya.yoshikawa@...il.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()

On 02/02/2012 04:44 PM, Takuya Yoshikawa wrote:
> 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?

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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