[concurrency-interest] Double Checked Locking in OpenJDK

Zhong Yu zhong.j.yu at gmail.com
Tue Aug 14 15:36:08 EDT 2012


>From a user's point of view, shouldn't constructors be special though?
An object shouldn't be considered in existence until its construction
is done; it is pathological that some outsider can observe a partially
constructed object. Life is simpler if we can eliminate that
possibility (unless `this` is leaked inside constructor)

Zhong Yu

On Tue, Aug 14, 2012 at 1:58 PM, Nathan Reynolds
<nathan.reynolds at oracle.com> wrote:
> 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 | Consulting Member of Technical Staff | 602.333.9091
> Oracle PSR Engineering | 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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