Incorrect validation of DST in java.util.SimpleTimeZone

[Peter Levart]

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.

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.

Venkat's patch therefore correctly fixes the remaining issue that is 
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.
>>>>>
>>>>
>>>
>>>
>>