Hi Peter,
On 15/11/2017 1:02 AM, Peter Levart wrote:
Hi David,
On 11/11/2017 07:51 AM, David Holmes wrote:
AFAICS SimpleTimeZone is simply not thread-safe. It has state that can
be modified concurrently without synchronization and with fields not
even declared volatile. Only the "cache" makes an attempt to use
synchronization. So clone() is never guaranteed to actually produce a
copy with valid/consistent field values.
The suggested patch certainly improves the situation by at least
resetting the cache of the cloned instance before returning it.
The instance of SimpleTimeZone that is shared among threads (internally
in JDK) is the defaultTimeZone instance (obtained through
package-private TimeZone.getDefaultRef() method). I checked the usages
and they seem to be "read-only" - not modifying the instance, just
obtaining information from it. The cache OTOH, as you say, is synchronized.
The initial problem statement was:
"When a thread is cloning a default timezone object (SimpleTimeZone) and
at the same time if a different thread modifies the time/year values, ..."
so that doesn't seem to be read-only. Though perhaps it is a very
specific race.
TimeZone and subclasses seem to be designed to be modified by single
thread only, but can be used from multiple threads to read the
information from them, including lazily computed and cached information.
Usage withing JDK seems to comply with that.
There's certainly no documentation of any such intent, or design. Seems
more like the synchronization has been added (or not) based on how it is
used within JDK rather than considering the actual API of the public types.
Venkat's patch therefore correctly fixes the remaining issue that is
Okay. As I said it certainly makes things better.
Cheers,
David
observed when the shared SimpleTimeZone instance is being cloned while
also being accessed from multiple threads in read-only mode.
Invalidating cache of the cloned instance just before returning it from
clone() method means that instance obtained from TimeZone.getDefault()
will never get cached state from original instance and will always have
to re-compute it, but I think this is still better than synchronizing on
the original instance which may never be optimized away (i.e. elided) by
JIT.
Regards, Peter
David
On 11/11/2017 3:53 PM, Venkateswara R Chintala wrote:
Thanks Sean. I am pasting the patch here:
--- old/src/java.base/share/classes/java/util/SimpleTimeZone.java
2017-11-11 11:17:38.643867420 +0530
+++ new/src/java.base/share/classes/java/util/SimpleTimeZone.java
2017-11-11 11:17:38.375870421 +0530
@@ -868,7 +868,11 @@
*/
public Object clone()
{
- return super.clone();
+ // Invalidate the time zone cache while cloning as it
+ // can be inconsistent due to race condition.
+ SimpleTimeZone tz = (SimpleTimeZone) super.clone();
+ tz.invalidateCache();
+ return tz;
}
/**
--- /dev/null 2017-11-02 17:09:59.155627814 +0530
+++ new/test/java/util/TimeZone/SimpleTimeZoneTest.java 2017-11-11
11:17:38.867864912 +0530
@@ -0,0 +1,55 @@
+/*
+ * @test
+ * @summary Tests the race condition between
java.util.TimeZone.getDefault() and java.util.GregorianCalendar()
+ * @run main SimpleTimeZoneTest
+*/
+
+import java.util.Calendar;
+import java.util.GregorianCalendar;
+import java.util.SimpleTimeZone;
+import java.util.TimeZone;
+
+public class SimpleTimeZoneTest extends Thread {
+ Calendar cal;
+
+ public SimpleTimeZoneTest (Calendar cal) {
+ this.cal = cal;
+ }
+
+ public static void main (String[] args) {
+ TimeZone stz = new SimpleTimeZone(7200000, "Asia/Jerusalem",
Calendar.MARCH, 27, 0, 3600000, Calendar.SEPTEMBER, 16, 0, 3600000);
+ TimeZone.setDefault(stz);
+
+ SimpleTimeZoneTest stt = new SimpleTimeZoneTest(new
GregorianCalendar());
+ stt.setDaemon(true);
+ stt.start();
+
+ for (int i = 0; i < 50000; i++) {
+ Calendar cal = new GregorianCalendar();
+ cal.clear();
+ cal.getTimeInMillis();
+ cal.set(2014, 2, 2);
+ cal.clear();
+ cal.getTimeInMillis();
+ cal.set(1970, 2, 2);
+ }
+
+ }
+
+ public void run() {
+ while (true) {
+ cal.setTimeZone(TimeZone.getDefault());
+ cal.clear();
+ cal.set(2008, 9, 9);
+ Calendar calInst = java.util.Calendar.getInstance();
+ calInst.setTimeInMillis(cal.getTimeInMillis());
+
+ if (calInst.get(java.util.Calendar.HOUR_OF_DAY) !=
cal.get(java.util.Calendar.HOUR_OF_DAY) ||
+ calInst.get(java.util.Calendar.MINUTE) !=
cal.get(java.util.Calendar.MINUTE) ||
+ calInst.get(java.util.Calendar.SECOND) !=
cal.get(java.util.Calendar.SECOND) ||
+ calInst.get(java.util.Calendar.MILLISECOND) !=
cal.get(java.util.Calendar.MILLISECOND)) {
+ throw new RuntimeException("Test failed");
+ }
+ }
+ }
+}
On 10/11/17 9:29 PM, Seán Coffey wrote:
I think the OpenJDK mailing lists accept attachments if in patch
format. If it's a simple short patch, it's acceptable to paste it
into email body.
Easiest solution is to use webrev[1]. If you can't upload this to
cr.openjdk.java.net - then one of your colleagues may be able to help.
[1] http://openjdk.java.net/guide/webrevHelp.html
Regards,
Sean.
On 10/11/17 12:18, Venkateswara R Chintala wrote:
Looks like the patch attached earlier is not visible. As this is my
first contribution, please let me know how I can send the patch for
review.
On 10/11/17 5:37 PM, Venkateswara R Chintala wrote:
Hi,
In a multi-threaded environment, when java.util.SimpleTimeZone
object is used to create a default timezone, there can be a race
condition between the methods java.util.Timezone.getDefault() and
java.util.Timezone.getDefaultRef() which can result in
inconsistency of cache that is used to validate a particular
time/date in DST.
When a thread is cloning a default timezone object
(SimpleTimeZone) and at the same time if a different thread
modifies the time/year values, then the cache values (cacheYear,
cacheStart, cacheEnd) can become inconsistent which leads to
incorrect DST determination.
We considered two approaches to fix the issue.
1)Synchronize access to cloning default timezone object when cache
is being modified.
2)Invalidate the cache while returning the clone.
We preferred the second option as synchronization is more expensive.
We have attached the patch and jtreg testcase. Please review.
