[jbosscache-dev] Fix for JBCACHE-1555

[Manik Surtani]

On 20 Jan 2010, at 11:20, Galder Zamarreno wrote:

> Hi,
> 
> Re: https://jira.jboss.org/jira/browse/JBCACHE-1555
> 
> I've got a fix for this issue that fixes both the writeSkew=true and
> writeSkew=false situations:
> 
> The problem as stated in 1555 is that
> MVCCNodeHelper.wrapNodeForWriting() peeks for data, via
> DC.peekInternalNodeAndDirectParent before acquiring any locks. So, if a
> thread ends up waiting for the lock and someone else comes and writes
> the data, when the waiting thread wakes up, it uses stale peeked data.
> 
> The solution I propose is the following:
> 
> if (lockForWriting && acquireLock(context, fqn)) {
>    needToCopy = true;
>    // re-peek in case we waited for a lock and some other thread
> modified the data while we're waiting
>    nodes = dataContainer.peekInternalNodeAndDirectParent(fqn,
> includeInvalidNodes);
> }
> n = nodeFactory.createWrappedNode(nodes[0], nodes[1]);
> 
> We only need to worry about the case where we wanna write cos on reads,
> we never have to acquire locks. So, on writes, after acquiring the lock,
> we re-peek to get the most up to date data, which we then use to create
> the wrapped node.
> 
> This seems to work fine for the test. We'd have to check hudson.
> 
> One doubt I have is whether the 1st peak here could be optimised to be
> an exist() call:
> 
> InternalNode[] nodes =
> dataContainer.peekInternalNodeAndDirectParent(fqn, includeInvalidNodes);
> InternalNode in = nodes[0];
> if (in != null) {
> ...
> 
> However, includeInvalidNodes might be key here and I don't see any
> exists in the DataContainer API that allows to pass that parameter. So,
> I think this needs to stay as a peek.

Yeah it can stay as a peek.  Compared to the cost of writing, an additional peek is negligible.

> 
> Thoughts?
> _______________________________________________
> jbosscache-dev mailing list
> jbosscache-dev at lists.jboss.org
> https://lists.jboss.org/mailman/listinfo/jbosscache-dev

--
Manik Surtani
manik at jboss.org
Lead, Infinispan
Lead, JBoss Cache
http://www.infinispan.org
http://www.jbosscache.org