[concurrency-interest] Double Checked Locking in OpenJDK

Stanimir Simeonoff stanimir at riflexo.com
Tue Aug 14 16:12:38 EDT 2012


On Tue, Aug 14, 2012 at 10:58 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 you greatly care about performance and since Java doesn't have struct
alikes (yet) and you are going to have tons of Point(s), just 2
double[]/int[] to reduce the cache misses/indirections - i.e. that's
another option how to organize your code.
OTOH if you choose more OO aproach it's still up to you - you have an
option to make it look "better" w/ final fields or forgo 'em potentionally
saving store-store on PowerPC.

That reminds me the keyword 'const' is unsed and potetially can be what
"final" is for fields but w/o memory/compiler barriers.

Stanimir

> >
> > 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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