Hi David,
On 11/11/17 13:37, David Holmes wrote:
Hi Peter,
On 11/11/2017 8:06 PM, Peter Levart wrote:
Hi Venkateswara R Chintala,
I would like to remind that TimeZone.clone() is also in the code path
of java.time.ZoneId.systemDefault() where it was relied on to be
optimized by JIT to never actually allocate the cloned TimeZone
object by employing escape analysis.
remind? Where is this documented?
Unfortunately it is not documented. My bad. I did test it at the time
and determined that ZoneId.systemDefault(), with my patch applied, did
not do allocations when JIT kicked-in. So we settled for an easier variant:
http://cr.openjdk.java.net/~plevart/jdk9-dev/ZoneId.systemDefault/webrev.04/
instead of a more complicated one that uses SharedSecrets to avoid cloning:
http://cr.openjdk.java.net/~plevart/jdk9-dev/ZoneId.systemDefault/webrev.01/
And given:
public static TimeZone getDefault() {
return (TimeZone) getDefaultRef().clone();
}
how can it not allocate??
When JIT compiles some method and inlines called methods, it also
analyses all allocations performed to see if some of them can be proved
to not escape the invocation of that compiled method. It then eliminates
allocations of such objects on heap and rather generates code that keeps
their state in registers or stack.
For example, take the following method:
String defaultTZID() {
return TimeZone.getDefault().getID();
}
When JIT compiles it and inlines invocations to other methods within it,
it can prove that cloned TimeZone instance never escapes the call to
defaultTZID() and can therefore skip allocating the instance on heap.
But this is fragile. If code in invoked methods changes, they may not
get inlined or EA may not be able to prove that the cloned instance
can't escape and allocation may be introduced. ZoneId.systemDefault() is
a hot method and it would be nice if we manage to keep it allocation free.
Regards, Peter
David
-----
It would be nice to verify if this patch keeps that behavior. To test
this, consider a JMH benchmark that simply invokes
ZoneId.systemDefault() and returns it from the test method. 1st
verify that unmodified code, when JITed, performs no allocations.
Then test with modified code by applying the patch and see if there's
any difference. Hint: use "-prof gc" command line options to run the
benchmarks.jar.
Regards, Peter
On 11/11/17 06:53, 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.
