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From "Andrea Reale (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-3162) Broken lock semantics in C client lock-recipe
Date Tue, 09 Oct 2018 15:19:00 GMT
Andrea Reale created ZOOKEEPER-3162:

             Summary: 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: 3.4.13, 3.0.0
            Reporter: Andrea Reale

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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