RFR: 8079315: UseCondCardMark broken in conjunction with CMS precleaning

Vitaly Davidovich vitalyd at gmail.com
Mon May 11 13:49:58 UTC 2015


Erik,

What would prevent compiler based reordering in your suggestions?

sent from my phone
On May 11, 2015 7:33 AM, "Erik Österlund" <erik.osterlund at lnu.se> wrote:

> Hi Andrew,
>
> > On 11 May 2015, at 11:58, Andrew Haley <aph at redhat.com> wrote:
> >
> > On 05/11/2015 11:40 AM, Erik Österlund wrote:
> >
> >> I have heard statements like this that such mechanism would not work
> >> on RMO, but never got an explanation why it would work only on
> >> TSO. Could you please elaborate?  I studied some kernel sources for
> >> a bunch of architectures and kernels, and it seems as far as I can
> >> see all good for RMO too.
> >
> > Dave Dice himself told me that the algorithm is not in general safe
> > for non-TSO.  Perhaps, though, it is safe in this particular case.  Of
> > course, I may be misunderstanding him.  I'm not sure of his reasoning
> > but perhaps we should include him in this discussion.
>
> I see. It would be interesting to hear his reasoning, because it is not
> clear to me.
>
> > From my point of view, I can't see a strong argument for doing this on
> > AArch64.  StoreLoad barriers are not fantastically expensive there so
> > it may not be worth going to such extremes.  The cost of a StoreLoad
> > barrier doesn't seem to be so much more than the StoreStore that we
> > have to have anyway.
>
> Yeah about performance I’m not sure when it’s worth removing these fences
> and on what hardware.
>
> In this case though, if it makes us any happier, I think we could probably
> get rid of the storestore barrier too:
>
> The latent reference store is forced to serialize anyway after the dirty
> card value write is observable and about to be cleaned. So the potential
> consistency violation that the card looks dirty and then cleaning thread
> reads a stale reference value could not happen with my approach even
> without storestore hardware protection. I didn’t give it too much thought
> but on the top of my mind I can’t see any problems. If we want to get rid
> of storestore too I can give it some more thought.
>
> But you know much better than me if these fences are problematic or not. :)
>
> Thanks,
> /Erik
>
> >
> > Andrew.
>
>
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