Re: [PATCH] kvm: get rid of srcu idx

[Gleb Natapov <gleb@redhat.com>]

On Mon, Oct 25, 2010 at 06:13:56PM -0200, Marcelo Tosatti wrote:
> On Mon, Oct 25, 2010 at 03:20:59PM +0200, Michael S. Tsirkin wrote:
> > On Mon, Oct 25, 2010 at 11:52:29AM -0200, Marcelo Tosatti wrote:
> > > On Sun, Oct 24, 2010 at 07:16:10PM +0200, Michael S. Tsirkin wrote:
> > > > srcu_idx is easy to misuse as it can not be used
> > > > in nested srcu_read_lock calls.  Switch to local
> > > > index to make correctness easier to verify.
> > > > 
> > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > 
> > > > ---
> > > 
> > > Looks good to me.
> > > 
> > > Not needed for -stable, i don't see a problem with the current
> > > code?
> > 
> > It's just that I can not convince myself current code never nestes
> > read_locks with srcu_idx.  I used to see host crashes under migration
> > stress they seem gone now but did not retest - could be just luck.
> 
> Well, you just changed where srcu index is saved. I don't see how it
> could make a difference in practice.
> 
If there is nested call to srcu read lock if srcu_idx is stored in vcpu
nested call will override previous srcu_idx value and srcu unlock will
not be called on it, but it will be called twice on new srcu_idx value.
If srcu_idx is saved on stack this will not happen, no?

--
			Gleb.