Benjamin Roth commented on CASSANDRA-12689:

I don't know if this is the reason for this issue but I guess there is a bug in 
Keyspace.apply. If aquiring lock for MV update with a future fails, the method 
wont return but still continue, even though the future completes with 
exception. I guess there is a simple "return" missing after 
future.completeExceptionally. In Trunk this is line 486.

                    if (!isClReplay && (System.currentTimeMillis() - 
mutation.createdAt) > DatabaseDescriptor.getWriteRpcTimeout())
                        logger.trace("Could not acquire lock for {} and table 
{}", ByteBufferUtil.bytesToHex(mutation.key().getKey()), 
                        Tracing.trace("Could not acquire MV lock");
                        if (future != null)
WriteTimeoutException(WriteType.VIEW, ConsistencyLevel.LOCAL_ONE, 0, 1));
                            throw new WriteTimeoutException(WriteType.VIEW, 
ConsistencyLevel.LOCAL_ONE, 0, 1);

Not only that the mutation is applied without a lock, wich certainly induces 
race conditions or corruptions, in the finally block the locks are unlocked but 
if lock has not been acquired, the corresponding entry in "locks" is null, thus 
throwing a NPE.

> All MutationStage threads blocked, kills server
> -----------------------------------------------
>                 Key: CASSANDRA-12689
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12689
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Local Write-Read Paths
>            Reporter: Benjamin Roth
>            Priority: Critical
> Under heavy load (e.g. due to repair during normal operations), a lot of 
> NullPointerExceptions occur in MutationStage. Unfortunately, the log is not 
> very chatty, trace is missing:
> 2016-09-22T06:29:47+00:00 cas6 [MutationStage-1] 
> org.apache.cassandra.concurrent.AbstractLocalAwareExecutorService Uncaught 
> exception on thread Thread[MutationStage-1,5,main]: {}
> 2016-09-22T06:29:47+00:00 cas6 #011java.lang.NullPointerException: null
> Then, after some time, in most cases ALL threads in MutationStage pools are 
> completely blocked. This leads to piling up pending tasks until server runs 
> OOM and is completely unresponsive due to GC. Threads will NEVER unblock 
> until server restart. Even if load goes completely down, all hints are 
> paused, and no compaction or repair is running. Only restart helps.
> I can understand that pending tasks in MutationStage may pile up under heavy 
> load, but tasks should be processed and dequeud after load goes down. This is 
> definitively not the case. This looks more like a an unhandled exception 
> leading to a stuck lock.
> Stack trace from jconsole, all Threads in MutationStage show same trace.
> Name: MutationStage-48
> State: WAITING on java.util.concurrent.CompletableFuture$Signaller@fcc8266
> Total blocked: 137  Total waited: 138.513
> Stack trace: 
> sun.misc.Unsafe.park(Native Method)
> java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
> java.util.concurrent.CompletableFuture$Signaller.block(CompletableFuture.java:1693)
> java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323)
> java.util.concurrent.CompletableFuture.waitingGet(CompletableFuture.java:1729)
> java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1895)
> com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly(Uninterruptibles.java:137)
> org.apache.cassandra.db.Mutation.apply(Mutation.java:227)
> org.apache.cassandra.db.Mutation.apply(Mutation.java:241)
> org.apache.cassandra.hints.Hint.apply(Hint.java:96)
> org.apache.cassandra.hints.HintVerbHandler.doVerb(HintVerbHandler.java:91)
> org.apache.cassandra.net.MessageDeliveryTask.run(MessageDeliveryTask.java:66)
> java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
> org.apache.cassandra.concurrent.AbstractLocalAwareExecutorService$FutureTask.run(AbstractLocalAwareExecutorService.java:162)
> org.apache.cassandra.concurrent.AbstractLocalAwareExecutorService$LocalSessionFutureTask.run(AbstractLocalAwareExecutorService.java:134)
> org.apache.cassandra.concurrent.SEPWorker.run(SEPWorker.java:109)
> java.lang.Thread.run(Thread.java:745)

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