[concurrency-interest] Double Checked Locking in OpenJDK

David Holmes davidcholmes at aapt.net.au
Mon Aug 13 01:13:38 EDT 2012


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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