[concurrency-interest] Double Checked Locking in OpenJDK

Zhong Yu zhong.j.yu at gmail.com
Wed Aug 15 19:58:57 EDT 2012


I'm not sure about the proper definition of safe/unsafe publication.

In any case, it's not easy to prove (on JMM rules) that the sync block
in the creation thread is ordered before the sync blocks in other
threads, so it's probably not worthwhile to contemplate such tricks.

On Wed, Aug 15, 2012 at 6:44 PM, David Holmes <davidcholmes at aapt.net.au> wrote:
> Zhong Yu writes:
>>
>> Any use case for unsafe publication of a mutable thread-safe object?
>>
>> If a use case does exist, there is a way to do it without
>> synchronizing constructor:
>>
>>     local_var = new Vector();  // not synchronized
>>     local_var.size();  // call a synchronized method
>>     shared_var = local_var;  // unsafe publication
>>
>> therefore synchronized constructor still isn't necessary.
>
> The above is turning unsafe publication into safe-publication.
>
> As I said in other email the whole issue here is to ensure that construction
> happens-before use. In the sharing case publication has to occur prior to
> use. That means you either tackle the problem directly, by ensuring
> construction happens-before use (ie with synchronized constructor on fully
> synchronized object). Or you establish that construction happens-before
> publication and publication happens-before use aka you use safe-publication
> mechanisms.
>
> David
> ------
>
>
>> Though, aesthetically, one might prefer to do synchronization in
>> constructors the same way it's done in instance methods, if the
>> overhead isn't a concern.
>>
>> Zhong Yu
>>
>> On Wed, Aug 15, 2012 at 2:56 PM, Ruslan Cheremin
>> <cheremin at gmail.com> wrote:
>> > Yes, it was an interesting discussion, and as result object with
>> > synchronized methods and constructor can be published unsafe (i.e.
>> > other thread can obtain reference to partially initialized object),
>> > but this patrially initialized state can't be observed, since all
>> > access going via sync-ed methods, which will block until
>> > initialization finish.
>> >
>> > So, staying away from the unsual and so strange and even ugly notation
>> > of locking in constructor, making object self-safe-publishing is not
>> > anything more complex, then making thread-safe method. Am I missed
>> > something?
>> >
>> > 2012/8/15 Zhong Yu <zhong.j.yu at gmail.com>:
>> >> I thought the conclusion of that thread is that synchronizing
>> >> constructor has the desired merit - if all constructors and methods
>> >> are synchronized, a non-creating thread won't observe the zero/partial
>> >> state of the object, even if the object reference is published
>> >> unsafely.
>> >>
>> >> (One guy, who shall remain nameless, muddied the water with some
>> >> mistaken statements of weaker memory guarantee. He has been corrected)
>> >>
>> >> Zhong Yu
>> >>
>> >> On Wed, Aug 15, 2012 at 11:58 AM, Yuval Shavit
>> <yshavit at akiban.com> wrote:
>> >>> 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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