[concurrency-interest] Double Checked Locking in OpenJDK

Zhong Yu zhong.j.yu at gmail.com
Tue Aug 14 20:46:23 EDT 2012


Lazy initialization is a common use case. Using thread-safe immutable
objects to avoid a `volatile` read is simple and well understood,
though it contains data race. I would say it is a common, useful and
justifiable data race.

You can argue that a volatile read isn't that expensive, lazy
initialization with volatile variable won't have any detectable
performance degrading on 99.99% systems. That may be true. But we
don't justify each choice of a concurrency tool by "profiling/testing"
real world scenario - seriously, who can afford that?

Zhong Yu

On Tue, Aug 14, 2012 at 7:01 PM, Boehm, Hans <hans.boehm at hp.com> wrote:
> Indeed.  My view is that we need special final field semantics primarily to
> deal with untrusted code, where you don’t have the control to guarantee
> there are no data races.  There has to be a way for untrusted code to pass a
> string to a library in a way that guarantees the string won’t change between
> the time a security manager looks at it and the time it’s used as a file
> name.  Thus we have to be able to construct classes that are fully
> immutable, no matter how badly the client mistreats them.  Final fields give
> you a way to do that without adding tons of overhead to a class like String.
>
>
>
> If you can avoid data races, do.
>
>
>
> Hans
>
>
>
> From: concurrency-interest-bounces at cs.oswego.edu
> [mailto:concurrency-interest-bounces at cs.oswego.edu] On Behalf Of Vitaly
> Davidovich
> Sent: Tuesday, August 14, 2012 3:36 PM
> To: Zhong Yu
> Cc: concurrency-interest at cs.oswego.edu
>
>
> Subject: Re: [concurrency-interest] Double Checked Locking in OpenJDK
>
>
>
> I think the lesson should be avoid data races - period. :) if you find a
> case where you can justify it (i.e. profiling/testing guided) then consider
> it.  I bet there will be few and far in between cases like that.
>
> Sent from my phone
>
> On Aug 14, 2012 5:02 PM, "Zhong Yu" <zhong.j.yu at gmail.com> wrote:
>
> 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
>>> >> >> >
>>> >> >> > _______________________________________________
>>> >> >> > Concurrency-interest mailing list
>>> >> >> > Concurrency-interest at cs.oswego.edu
>>> >> >> > http://cs.oswego.edu/mailman/listinfo/concurrency-interest
>>> >> >> >
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>>> >> >> >
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>>> >> >> >
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>>> >> >> >
>>> >> >> >
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