[concurrency-interest] Double Checked Locking in OpenJDK

Ruslan Cheremin cheremin at gmail.com
Wed Aug 15 12:49:30 EDT 2012


> 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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