Re: [infinispan-dev] AtomicHashMap concurrent modifications in pessimistic mode
On 16 May 2013, at 15:04, Dan Berindei <dan.berindei(a)gmail.com> wrote:
Hi guys
I'm working on an intermittent failure in NodeMoveAPIPessimisticTest and I think
I've come across what I think is underspecified behaviour in AtomicHashMap.
Say we have two transactions, tx1 and tx2, and they both work with the same atomic map in
a pessimistic cache:
1. tx1: am1 = AtomicMapLookup.get(cache, key)
2. tx2: am2 = AtomicMapLookup.get(cache, key)
3. tx1: am1.put(subkey1, value1) // locks the map
4. tx2: am2.get(subkey1) // returns null
5. tx1: commit // the map is now {subkey1=value1}
6. tx2: am2.put(subkey2, value2) // locks the map
7. tx2: commit // the map is now {subkey2=value2}
It's not clear to me from the AtomicMap/AtomicHashMap javadoc if this is ok or if
it's a bug...
If optimistic, step 7 should fail with a write skew check. If pessimistic, step 2 would
*usually* block assuming that another thread is updating the map, but since neither tx1 or
tx2 has started updating the map yet, neither has a write lock on the map. So that
succeeds. I'm not sure if this is any different from not using an atomic map:
1. tx1: cache.get(k, v); // reads into tx context
2. tx2: cache.get(k, v);
3. tx1: cache.put(k, v + 1 );
4. tx1: commit
5. tx2: cache.put(k, v + 1 );
6. tx2: commit
here as well, if using optimistic, step 6 will fail with a WSC but if pessimistic this
will work (since tx2 only requested a write lock after tx1 committed/released its write
lock).
Note that today the map is overwritten by tx2 even without step 4
("tx2: am2.get(subkey1)"). I'm pretty sure that's a bug and I fixed it
locally by using the FORCE_WRITE_LOCK in AtomicHashMapProxy.getDeltaMapForWrite.
However, when the Tree API moves a node it first checks for the existence of the
destination node, which means NodeMoveAPIPessimisticTest is still failing. I'm not
sure if I should fix that by forcing a write lock for all AtomicHashMap reads, for all
TreeCache reads, or only in TreeCache.move().
I think only in TreeCache.move()
--
Manik Surtani
manik(a)jboss.org
twitter.com/maniksurtani
Platform Architect, JBoss Data Grid
http://red.ht/data-grid
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