[infinispan-dev] eager locking and deadlocks

[Manik Surtani]

On 6 Jul 2010, at 16:06, Mircea Markus wrote:

> Hi,
> 
> Whit eager locking enabled, cache.put(k,v) acquires locks as follows:
> 1. acquire local lock on k
> 2. acquire remote lock on k
> 3. if 2 fails release local lock on k
> 
> Previous sequence (i.e. local then remote) makes more sense to me than the reversed:
> 1. acquire remote lock on k
> 2. acquire local lock on k
> 3. if 2 fails release remote lock on k 
> 
> This is because it doesn't make sense to go remotely just then to realise that you cannot acquire a local lock.
> 
> However, this approach does not work with deadlock detection[1] which requires the 2nd approach.
> Here are my options:
> 1. forbid deadlock detection on eager locking
> 2. allow users to use deadlock detection with eager locking and if DLD is on to use the 2nd locking approach, otherwise to use the first one. 
> 3.?

Would this not result in a very complex implementation though?

> 
> 2 is doable, and I think DLD might have a good impact on eager locking, as in this situation the transactions hold locks longer, hence the chance for deadlock is higher. 
> 
> Wdyt?
> 
> Cheers,
> Mircea
> 
> [1] here is why:
> tx1 and tx2 two transactions.
> tx1 originates on node A, tx2 originates on node B
> 
> Step 1 (simultaneously):
> tx1: A.put(k1) -> lock(tx1) = {A_k1, B_k1}
> tx2: B.put(k2) -> lock(tx2) = {A_k2, B_k2}
> 
> Step 2:
> tx1: A.put(k2) -> status(tx1) = Holds lock on {A_k1, B_k1} and tries to lock A_k2 
> tx2: B.put(k1) -> status(tx2) = Holds lock on {A_k2, B_k2} and tries to lock A_k1
> 
> DLD on Tx1 cannot progress: it tries to lock A_k2 and cannot, it is aware that A_k2 is locked by tx2, but it doesn't know that tx2's intention is to lock A_k1(that would mean a deadlock). Why? because tx2 hasn't replicated its intention to the other node as it's waiting to acquire lock on A_k1.
> Similar to Tx2.
> 
> 
> 
> 
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--
Manik Surtani
manik at jboss.org
Lead, Infinispan
Lead, JBoss Cache
http://www.infinispan.org
http://www.jbosscache.org