[concurrency-interest] Double Checked Locking in OpenJDK

Yuval Shavit yshavit at akiban.com
Wed Aug 15 12:58:45 EDT 2012


There was a discussion here a few months ago about synchronizing
constructors -- I had asked why it's not allowed, and the discussion hit on
some of the similar points brought up in this thread.

But to your point specifically, synchronizing a constructor (via
"synchronized(this) {...}" surrounding its body) still doesn't give you
full thread safety (even assuming immutability after the constructor -- but
without final fields). It ensures that a thread can observe the object
either fully constructed *or* with all its fields having their default
values. In other words, even if your constructor is synchronized on the
same object your getter is, a thread could observe a field as it was before
the constructor was invoked.

http://markmail.org/message/mav53xzo4bqu7udw

On Wed, Aug 15, 2012 at 12:49 PM, Ruslan Cheremin <cheremin at gmail.com>wrote:

> > The reason to keep them distinct is because in general the mechanisms for
> > safe publication are external to the class, while those for thread-safety
> > are internal. It is only an edge case where use of synchronized in a
> > constructor can achieve safe-publication.
>
> Well, actually I do not understand your point. If I use some kind of
> synchronization to make methods of my object thread-safe -- can't I
> also apply same thing to constructor? For me, it makes the thing only
> clearer. Object can be thread-safe -- and it is totally thread safe.
> Object can require external synchronization for correct multithreaded
> use -- and it requires the sync for publishing and for usage also.
>
> From my point of view, the distinction you talking about is more
> historically reasoned. "Sync method if you want it to be thread-safe"
> is commonly learned mantra, but "take care of initialization also" is
> not so common. More information about it, more education, more
> different code samples with outlined "here is the dragons" will change
> the situation, I sure, it just have to be highlighted more often.
>
>
> > People have to recognize that sharing an object requires shared mutable
> > state, and the number one tenet of concurrent programming is that access
> to
> > shared mutable state has to be synchronized (in a general sense not
> > specifically use of 'synchronized' keyword).
> >
> > Making every object safely publishable could be done, but for 99% of
> objects
> > it would be a waste of effort. Programs without data races don't have
> issues
> > with unsafe publication.
> >
> > David
> >
> > -----Original Message-----
> > From: concurrency-interest-bounces at cs.oswego.edu
> > [mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of Nathan
> > Reynolds
> > Sent: Wednesday, 15 August 2012 4:59 AM
> > To: concurrency-interest at cs.oswego.edu
> > Subject: Re: [concurrency-interest] Double Checked Locking in OpenJDK
> >
> > 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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