[concurrency-interest] Double Checked Locking in OpenJDK

Nathan Reynolds nathan.reynolds at oracle.com
Tue Aug 14 14:58:56 EDT 2012


We seem to be splitting two notions (i.e thread-safe and safe 
publication) when they should be combined in a sense.  Typically, when 
we say thread-safe we talk about the operations performed on the object 
after it was constructed (and its contents are globally visible).  
However, we need to consider that executing the constructor is modifying 
the state of the object.  It requires the same mechanisms that the rest 
of the class uses to ensure thread-safety.  Even though, there is only 1 
thread executing the constructor, a proper releasing of a lock or some 
other happens-before construct is required to ensure that the memory 
updates by the thread are made globally visible before the object is 
accessed by another thread. This is what we are calling safe 
publication.  So, safe publication is a subset of thread-safety except 
it is limited to what happens after the constructor is called and before 
the object is used by multiple threads.

A beautifully-written class can be thread-safe with respect to calling 
its member methods but not thread-safe with respect to calling its 
constructor.  It is this latter case that many stumble upon because they 
think that constructors are inherently thread-safe because they are 
executed single-threadedly.  What they fail to realize is that the 
execution of a constructor can overlap with the execution of other code 
from the view point of what is happening in memory.  This same problem 
applies to more rare case of regular methods which can be proven to 
execute in a single thread but don't use synchronization before multiple 
threads start accessing the shared data.

Nathan Reynolds 
<http://psr.us.oracle.com/wiki/index.php/User:Nathan_Reynolds> | 
Consulting Member of Technical Staff | 602.333.9091
Oracle PSR Engineering <http://psr.us.oracle.com/> | Server Technology
On 8/13/2012 4:08 PM, David Holmes wrote:
> Ruslan Cheremin writes:
>> For me it is confusing: java has only one way to have really immutable
>> object, and this way also gives you a total thread safety even for
>> data race based publication. But then docs refer object as "immutable
>> and thread-safe" -- we still can't assume it to be really thread-safe?
> It is better/simpler to isolate the notion of thread-safety and safe
> publication. Thread-safety comes into play after you have safely shared an
> object. The means by which you safely share an object is orthogonal to how
> the object itself is made thread-safe.
>
> The means by which an object is shared has to involve shared mutable state,
> and use of shared mutable state always needs some form of synchronization
> (either implicit eg due to static initialization; or explicit by using
> volatile or synchronized getter/setter methods).
>
> David
> -----
>
>> It's a pity, especially because true immutability gives us some
>> chances of performance optimization. As in this case -- we do not
>> really need .path to be volatile here, if we would assume Path to be
>> truly immutable. volatility here required only for ensuring safe
>> publishing.
>>
>> 2012/8/13 David Holmes <davidcholmes at aapt.net.au>:
>>> Ruslan Cheremin writes:>
>>>> But is there a way to define "safe for data race publishing"? I as
>>>> far, as I remember, "immutable and thread-safe" is standard mantra in
>>>> JDK javadocs for totally safe objects. j.l.String has same mantra --
>>>> and it is safe for any way of publishing. Does you mean, I should
>>>> explicitly add "safe even for publishing via data race" in docs? But I
>>>> can't remember any such phrase in JDK docs.
>>> I don't recall anything in the JDK docs that mention being
>> "totally safe"
>>> regardless of publication mechanism. Some classes, eg String, have been
>>> defined such that they do have that property (for security reasons). In
>>> general neither "thread-safe" nor "immutable" imply
>>> safe-for-unsynchronized-publication.
>>>
>>> Java Concurrency In Practice (jcip.net) does define additional potential
>>> annotations, where @Immutable would indeed capture the requirement of
>>> safe-for-unsynchronized-publication.
>>>
>>> David
>>> -----
>>>
>>>> 2012/8/13 David Holmes <davidcholmes at aapt.net.au>:
>>>>> Ruslan Cheremin writes:
>>>>>> Well, Path javadoc explicitly says "immutable and safe for
>>>>>> multithreaded use". Although it is not strictly defined in java what
>>>>>> exactly means "safe for multithreaded use" -- does it mean safe for
>>>>>> publishing via data race, among others? -- I suppose, it
>> should be. Am
>>>>>> I wrong here?
>>>>> "safe for multi-threaded use" does not generally imply that it
>>>> is safe to
>>>>> publish instances without synchronization of some form.
>>>>>
>>>>> David
>>>>> -----
>>>>>
>>>>>>  From other side, File.toPath javadoc explicitly says what "returned
>>>>>> instance must be the same for every invocation", so sync block is
>>>>>> required here for mutual exclusion on initialization phase. Without
>>>>>> this requirement it is also safe to live without sync block, afaik.
>>>>>>
>>>>>> 2012/8/13 David Holmes <davidcholmes at aapt.net.au>:
>>>>>>> Ruslan Cheremin writes:
>>>>>>>> First of all, Path is immutable, so DCL is safe here even without
>>>>>>>> volatile. Volatile here is not required from my point of view.
>>>>>>> Without the volatile the Path implementation (Path is an
>>>>>> interface) must be
>>>>>>> such that an instance of Path can be safely published without
>>>>>> any additional
>>>>>>> forms of synchronization. Immutability does not in itself
>>>>>> ensure that. You
>>>>>>> would have to examine the actual implementation class.
>>>>>>>
>>>>>>> David Holmes
>>>>>>> ------------
>>>>>>>
>>>>>>>>
>>>>>>>> 2012/8/12 Dmitry Vyazelenko <vyazelenko at yahoo.com>:
>>>>>>>>> Hi Richard,
>>>>>>>>>
>>>>>>>>> The variable "filePath" is volatile, so the double-checked
>>>>>>>> locking is correct in this case. It would have been a bug
>>>>>> prior to Java 5.
>>>>>>>>> Best regards,
>>>>>>>>>
>>>>>>>>> Dmitry Vyazelenko
>>>>>>>>>
>>>>>>>>> On Aug 12, 2012, at 21:35 , Richard Warburton
>>>>>>>> <richard.warburton at gmail.com> wrote:
>>>>>>>>>> Hello,
>>>>>>>>>>
>>>>>>>>>> The current implementation of java.io.File::toPath [0]
>>>> appears to be
>>>>>>>>>> using the double checked locking pattern:
>>>>>>>>>>
>>>>>>>>>>      public Path toPath() {
>>>>>>>>>>          Path result = filePath;
>>>>>>>>>>          if (result == null) {
>>>>>>>>>>              synchronized (this) {
>>>>>>>>>>                  result = filePath;
>>>>>>>>>>                  if (result == null) {
>>>>>>>>>>                      result =
>>>> FileSystems.getDefault().getPath(path);
>>>>>>>>>>                      filePath = result;
>>>>>>>>>>                  }
>>>>>>>>>>              }
>>>>>>>>>>          }
>>>>>>>>>>          return result;
>>>>>>>>>>      }
>>>>>>>>>>
>>>>>>>>>> I was going to report the bug, but I'm a little
>> uncertain of the
>>>>>>>>>> interaction between the local variable 'result' and DCL
>>>> since I've
>>>>>>>>>> previously only seen the checking condition on the
>> shared field
>>>>>>>>>> itself.  Can someone here either confirm that its a bug or
>>>>>> explain how
>>>>>>>>>> the 'result' variable is fixing things?
>>>>>>>>>>
>>>>>>>>>> regards,
>>>>>>>>>>
>>>>>>>>>>   Richard
>>>>>>>>>>
>>>>>>>>>> [0] See the end of
>>>>>>>>>>
>> hg.openjdk.java.net/jdk8/jdk8/jdk/file/da8649489aff/src/share/clas
>>>>>>>> ses/java/io/File.java
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