Alex Parvulescu commented on OAK-4824:

bq. This is a regression introduced with OAK-4746: the method 
TarWriter.createNextGeneration() seems over eagerly synchronized.
right. looks bad. the issue is with accessing the {{access}} var which seems to 
always be protected by a lock on the {{TarWriter}} itself. should we wrap only 
the {{if (access == null)}} check in a {{synchronized (this)}} block?

> Deadlock in TarWriter
> ---------------------
>                 Key: OAK-4824
>                 URL: https://issues.apache.org/jira/browse/OAK-4824
>             Project: Jackrabbit Oak
>          Issue Type: Bug
>          Components: segment-tar
>            Reporter: Michael Dürig
>            Priority: Critical
>              Labels: deadlock, resilience, threading
>             Fix For: Segment Tar 0.0.14
> There is the potential for a deadlock between concurrent calls to 
> {{TarWriter#createNextGeneration()}} and {{TarWriter#flush()}}: both methods 
> try to acquire a lock on this and another lock on {{TarWriter.file}} but in 
> different order. I observed the deadlock when running 
> {{CompactionAndCleanupIT.randomAccessFileConcurrentReadAndLength()}}.
> This is a regression introduced with OAK-4746: the method 
> {{TarWriter.createNextGeneration()}} seems over eagerly synchronized. I would 
> argue that we could drop synchronization for that method entirely as the part 
> after the call to {{close()}} will in any case only ever be executed once by 
> a single thread. All other threads will fail with an 
> {{IllegalStateException}}).
> [~alexparvulescu], WDYT?

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