[concurrency-interest] Double Checked Locking in OpenJDK

Zhong Yu zhong.j.yu at gmail.com
Tue Aug 14 17:02:56 EDT 2012


Lesson I learned from this thread:

do *not* write thread-safe immutable class, unless there's a good reason.

One good reason is double checked locking without volatile.

Zhong Yu

On Tue, Aug 14, 2012 at 3:09 PM, Vitaly Davidovich <vitalyd at gmail.com> wrote:
> You should use final for its semantic meaning/code clarity unless you
> yourself are publishing it unsafely in your library.  If users of your lib
> are sharing this class, they need to provide the required
> synchronization/hand off.  That's my personal view (and I always try to use
> final as much as possible, but mostly for clarity/code semantics).
>
> For people running the jvm on an arch where final is not just compiler
> barrier, I agree it's a bit of a problem - really, final should not carry
> both java semantic and JMM meaning, but I can see why that was done.
>
> Sent from my phone
>
> On Aug 14, 2012 3:58 PM, "Zhong Yu" <zhong.j.yu at gmail.com> wrote:
>>
>> On Tue, Aug 14, 2012 at 2:52 PM, Vitaly Davidovich <vitalyd at gmail.com>
>> wrote:
>> > Yes, presumably on some archs with very weak memory order it could cause
>> > some performance impact.  On TSO, it's just a compiler barrier
>> > preventing
>> > code motion that would publish the reference before constructor
>> > completes,
>> > but on those weak archs it could also mean a hardware fence.  Whether
>> > the
>> > tradeoff is worth it in general or not is debatable.  You can't have it
>> > both
>> > ways, I agree, but the jvm and JMM give you options and guidance on how
>> > to
>> > do the "right" thing.
>>
>> If I'm writing a Java library, containing a Point(x,y) class, which is
>> immutable (in the narrower sense), should I use final fields or not?
>> What's the "right" thing to do?
>>
>> If the answer is platform dependent, oh well.
>>
>> Zhong Yu
>>
>> >
>> > Sent from my phone
>> >
>> > On Aug 14, 2012 3:47 PM, "Zhong Yu" <zhong.j.yu at gmail.com> wrote:
>> >>
>> >> Are `final` fields a problem to compiler optimization?
>> >>
>> >> You can't have both ways.
>> >>
>> >> Either `final` field semantics is good and cheap, therefore it should
>> >> be extended to all fields.
>> >>
>> >> Or there is some downsides with 'final' field semantics, so we
>> >> shouldn't encourage people to apply `final` whenever they can. Lets
>> >> hear those downsides.
>> >>
>> >> Zhong Yu
>> >>
>> >> On Tue, Aug 14, 2012 at 2:42 PM, Vitaly Davidovich <vitalyd at gmail.com>
>> >> wrote:
>> >> > This is only an issue when publishing unsafely.  Allowing treating
>> >> > constructors as regular methods is a good thing as it gives the
>> >> > compiler
>> >> > a
>> >> > chance to optimize code, which everyone likes and benefits from.
>> >> >
>> >> > Sent from my phone
>> >> >
>> >> > On Aug 14, 2012 3:39 PM, "Zhong Yu" <zhong.j.yu at gmail.com> wrote:
>> >> >>
>> >> >> 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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>> >> >> >
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>> >> >> >
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>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> > _______________________________________________
>> >> >> > Concurrency-interest mailing list
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>> >> >> > http://cs.oswego.edu/mailman/listinfo/concurrency-interest
>> >> >> >
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