[concurrency-interest] Double Checked Locking in OpenJDK

Yuval Shavit yshavit at akiban.com
Mon Aug 13 15:51:36 EDT 2012


This class is immutable but not thread-safe absent safe publication:

public class MyImmutable {
    private long val; // not marked final or volatile
    public MyImmutable(long val) {
        this.val = val;
    }

    public int getValue() {
        return val;
    }
}

In the absence of safe publication, a thread is allowed to see:

    - val's default value (0)
    - the value passed to the constructor
    - a word tear in which val contains only the high or low word from the
value passed to the constructor

On Mon, Aug 13, 2012 at 3:39 PM, Ruslan Cheremin <cheremin at gmail.com> wrote:

> 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'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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> >> >> >> >
> >> >> >> >
> >> >> >> > _______________________________________________
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> >> >>
> >> >
> >>
> >
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