[concurrency-interest] Double Checked Locking in OpenJDK

Dr Heinz M. Kabutz heinz at javaspecialists.eu
Mon Aug 13 01:41:11 EDT 2012


The j.l.String class is an interesting example to use.  It is thread
safe, but it contains a benign data race in the hashCode() method.
The class contains mutable data, without any synchronisation, yet is
still thread safe.

Heinz

On 13/08/2012, David Holmes <davidcholmes at aapt.net.au> wrote:
> 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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-- 
Dr Heinz M. Kabutz (PhD CompSci)
Author of "The Java(tm) Specialists' Newsletter"
Sun Java Champion
IEEE Certified Software Development Professional
http://www.javaspecialists.eu
Tel: +30 69 75 595 262
Skype: kabutz


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