Hiroshi Ikeda created HADOOP-10524:
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Summary: Race condition around MutableMetric and its subclasses
Key: HADOOP-10524
URL: https://issues.apache.org/jira/browse/HADOOP-10524
Project: Hadoop Common
Issue Type: Bug
Reporter: Hiroshi Ikeda
Priority: Minor
For example, MutableGaugeInt has two methods:
{code}
public synchronized void incr() {
++value;
setChanged();
}
public void set(int value) {
this.value = value;
setChanged();
}
{code}
and the lack of synchronization of the {{set}} method causes a problem that
calling the {{set}} method might be ignored while another thread is calling the
{{incr}} method, such as:
(1) Thread1 takes the current value in the {{incr}} method.
(2) Thread2 sets the new value in the {{set}} method.
(3) Thread1 adds +1 to the taken value and sets the value in the {{incr}}
method.
Also, in the first place, MutableMetric has a volatile instance variable
{{changed}}, but lack of synchronization causes a problem that it drops the
latest notification which is called just before clearing the {{changed}}
variable. That means, the volatile keyword is useless unless it is needed to
just check the flag itself. Indeed, the implementation of the method
{{snapshot}} in MutableCounterInt has this problem because of lack of
synchronization.
Anyway, synchronization around MutableMetric and its subclasses is doubtful and
should be reviewed.
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