[concurrency-interest] Double Checked Locking in OpenJDK

Vitaly Davidovich vitalyd at gmail.com
Tue Aug 14 15:52:13 EDT 2012


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.

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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