[concurrency-interest] Realistic expectations of AtomicLongFieldUpdater

David Holmes davidcholmes at aapt.net.au
Wed Sep 20 21:50:35 EDT 2017


If you use more than one updater to update the same field, or you mix updater use with raw access, then there is no guarantee of atomicity.

 

For initialization, as long as you don’t publish “this” before construction is complete, there can be no atomicity issues because there is no concurrency.

 

David

 

From: Concurrency-interest [mailto:concurrency-interest-bounces at cs.oswego.edu] On Behalf Of Carl Mastrangelo
Sent: Thursday, September 21, 2017 11:31 AM
To: dholmes at ieee.org
Cc: concurrency-interest at cs.oswego.edu
Subject: Re: [concurrency-interest] Realistic expectations of AtomicLongFieldUpdater

 

I still don't understand.  I have included a copy of the javadoc I am reading:

 

"""

Note that the guarantees of the compareAndSet method in this class are weaker than in other atomic classes. Because this class cannot ensure that all uses of the field are appropriate for purposes of atomic access, it can guarantee atomicity only with respect to other invocations of compareAndSet and set on the same updater.

"""

 

So here is my reasoning:

 

1.  Modifications of a variable using the updater are atomic if and only if using set() or compareAndSet()

2.  Volatile variable initialization is not done using the set() or compareAndSet()

3.  Therefore,  compareAndSet is not atomic.

 

The conclusion seems absurd, but follows the javadoc.  What am I missing?

 

 

 

 

On Tue, Sep 19, 2017 at 1:40 PM, David Holmes <davidcholmes at aapt.net.au <mailto:davidcholmes at aapt.net.au> > wrote:

I don’t agree the Javadoc implies that. As I said the issue is atomicity not visibility – the field is volatile. At initialization you have not published the instance and so atomicity is not an issue (not that it would generally be an issue anyway – all loads and stores have to be atomic as it is volatile).

 

Cheers,

David

 

From: Carl Mastrangelo [mailto:notcarl at google.com <mailto:notcarl at google.com> ] 
Sent: Wednesday, September 20, 2017 6:33 AM
To: dholmes at ieee.org <mailto:dholmes at ieee.org> 
Cc: concurrency-interest at cs.oswego.edu <mailto:concurrency-interest at cs.oswego.edu> 
Subject: Re: [concurrency-interest] Realistic expectations of AtomicLongFieldUpdater

 

I left publishing out on purpose, though the non static field updater was a typo.   

 

The origin of my question was if I needed to use the updater for field initialization.   The javadoc implies yes, but it feels incorrect.  

 

On Mon, Sep 18, 2017 at 1:53 PM, David Holmes <davidcholmes at aapt.net.au <mailto:davidcholmes at aapt.net.au> > wrote:

The issue is with atomicity not visibility. The volatile semantics of the field ensure visibility provided the Foo instance is not published during construction. Your example doesn’t show how the Foo instance is published. (And your updater reference would normally be a static).

 

The atomicity caveat is just to account for potential interactions between raw updates and updates via the field updater. Though to be honest I’m having trouble recalling exactly what the issue is.

 

David

 

From: Concurrency-interest [mailto:concurrency-interest-bounces at cs.oswego.edu <mailto:concurrency-interest-bounces at cs.oswego.edu> ] On Behalf Of Carl Mastrangelo
Sent: Tuesday, September 19, 2017 3:57 AM
To: concurrency-interest at cs.oswego.edu <mailto:concurrency-interest at cs.oswego.edu> 
Subject: [concurrency-interest] Realistic expectations of AtomicLongFieldUpdater

 

Hi concurrency-interest,

 

The classes AtomicLongFieldUpdater, AtomicReferenceFieldUpdater, and AtomicIntegerFieldUpdater all have the same ominous comment at the top:

 

 * <p>Note that the guarantees of the {@code compareAndSet}

 * method in this class are weaker than in other atomic classes.

 * Because this class cannot ensure that all uses of the field

 * are appropriate for purposes of atomic access, it can

 * guarantee atomicity only with respect to other invocations of

 * {@code compareAndSet} and {@code set} on the same updater.

 

 

I am not sure how the Java memory model describes what happens to field updaters, especially in regards to class initialization.  For example consider the following:

 

 

class Foo {

  AtomicIntegerFieldUpdater<Foo> updater = AtomicIntegerFieldUpdater.newUpdater(Foo.class, "field");

  private volatile int field = 1;

 

  public void bar() {

    updater.compareAndSet(this, 1, 2);

  }

}

 

If two threads try to access field:

 

T1: final Foo f = new Foo();

T2: f.bar();

 

Is is guaranteed that subsequent reads of field will be 2?   From the docs it implies that it may fail, since field was not initialized using updater.set().  It my reading correct?

 

 

After talking to Martin Buccholz, it seems like even reads should be done using the updater, but doesn't this violate the semantics of volatile?

 

 

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