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https://issues.apache.org/jira/browse/ZOOKEEPER-3162?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Andor Molnar resolved ZOOKEEPER-3162.
-------------------------------------
Resolution: Fixed
Fix Version/s: 3.5.5
3.4.14
3.6.0
Issue resolved by pull request 699
[https://github.com/apache/zookeeper/pull/699]
> Broken lock semantics in C client lock-recipe
> ---------------------------------------------
>
> Key: ZOOKEEPER-3162
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3162
> Project: ZooKeeper
> Issue Type: Bug
> Components: c client
> Affects Versions: 3.0.0, 3.4.13
> Reporter: Andrea Reale
> Priority: Major
> Labels: pull-request-available
> Fix For: 3.6.0, 3.4.14, 3.5.5
>
> Time Spent: 2h 40m
> Remaining Estimate: 0h
>
> As reported (but never fixed) in the past by ZOOKEEPER-2409, ZOOKEEPER-2038
> and (partly) ZOOKEEPER-2878, the C client lock-recipe implementation is
> broken.
> I identified three issues.
> The main one (as also reported in the aforementioned reports) is that the
> logic that goes through the lock waiting list is broken. child_floor uses
> strcmp and compares the full node name (i.e., sessionID-sequence) rather than
> only comparing the sequence number. This makes it possible for two different
> clients to hold the lock at the same time: assume two clients, one associated
> with session A, the other with session B, with A < B lexicographically. Now
> assume that at some point a thread in B holds a lock and a thread in A tries
> to acquire the same lock. A will manage to get the lock because of the wrong
> comparison function, so now two guys hold the lock.
> The second issue is a possible deadlock inside zkr_lock_operation.
> zkr_lock_operation is always called by holding the mutex associated to the
> client lock. In some cases, zkr_lock_operaton may decide to give-up locking
> and call zkr_lock_unlock to release the lock. When this happens, it will try
> to acquire again the same phtread mutex, which will lead to a deadlock.
> The third issue relates to the return value of zkr_lock_lock. According to
> the API docs, the functions returns 0 when no errors. Then it is up to the
> invoker to check when the lock is held by calling zkr_lock_isowner. However,
> the implementation, in case of no error, returns zkr_lock_isowner. This is
> wrong because it becomes impossible to distinguish an error condition from a
> success (but not ownerhsip). Instead the API (as described in the docs, btw)
> should return always 0 when no errors occur.
> Shortly I will add the link to a PR fixing the issues.
>
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