Re: [infinispan-dev] eager locking and deadlocks
On 6 Jul 2010, at 19:40, Manik Surtani wrote:
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?
the
InterceptorChainFactory would add a parameter to the LockInterceptor, a boolean value.
Then there would be an if in visitLockControlCommand.
The bit I don't know about is to rollback the remote lock acquisition if I cannot
acquire it locally (remember remote lock is acquired when trying to acquire local one).
Vladimir - do I have support for release a remote lock?
>
> 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(a)jboss.org
Lead, Infinispan
Lead, JBoss Cache
http://www.infinispan.org
http://www.jbosscache.org
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