RFR: JDK-8227831: Avoid using volatile for write-once, read-many class field

Mandy Chung mandy.chung at oracle.com
Thu Jul 18 17:58:50 UTC 2019

JDK-8219774 is relevant to this patch (this was discussed in the code
review for JDK-8219378: NPE in ReflectionFactory.newMethodAccessor
when langReflectAccess not initialized).

This cleans up the initialization of LangReflectAccess:

I moved LangReflectAccess to jdk.internal.access to be consistent with
other *Access classes (LangReflectAccess was added before the common
SharedSecrets class was introduced).   AccessibleObject was initialized
early during startup and this patch causes initializing SharedSecret and
LangReflectAccess earlier.  This is not an official review for JDK-8219774
but I'm interested if this improves the performance comparing with
Ogata's patch.

Ogato - can you help running the micro benchmark you have with
the patch for JDK-8219774?


On 7/18/19 2:53 AM, Claes Redestad wrote:
> Hi David,
> On 2019-07-18 06:26, David Holmes wrote:
>> Hi Claes,
>> On 18/07/2019 1:04 am, Claes Redestad wrote:
>>> Hi,
>>> removing volatile aligns LangReflectAccess initialization with the
>>> pattern used for other access objects: it's only initialized in the
>>> static initializer of some class which we ensure is initialized, which
>>> means any initialization race is guarded by the initialization of said
>>> class - in this case j.l.r.Modifier.
>> If the field is not volatile then we are not guaranteed to correctly 
>> see the constructed LangReflectAccess instance. Without volatile (or 
>> without additional use of unconditional sync in the accessor) we do 
>> not have a happens-before relationship between the construction of 
>> the LRA instance, and the setting of the field >
>>> Initialization of other access objects are not guarded by any
>>> explicit synchronization, however, since similar load/store barriers
>>> will be in effect by ensuring the class that constructs the object has
>>> been initialized. So I think you could remove the explicit
>>> synchronization.
>> We are not guaranteed to hit the class initialization checks that 
>> would guarantee the necessary ordering.
> You better not look at the existing code in
> jdk.internal.access.SharedSecrets :-)
> Would the unsafe.ensureClassInitialized(..) pattern used there bring
> stronger guarantees? That would be the pattern I'm suggesting the code
> under review here to align with.
> And for the record none of the *Access objects carry state and are
> strictly delegating to either static methods/constructors or instance
> methods on their arguments.
> /Claes
>> David
>> -----
>>> I'm not sure why LangReflectAccess has not moved in with other
>>> SharedSecrets. Perhaps this is just an artefact of having been around
>>> for a long time, but maybe that'd cause some cyclic bootstrap
>>> dependency. Either way it's nice to see it align to use the same
>>> pattern.
>>> Thanks!
>>> /Claes
>>> On 2019-07-17 10:00, Kazunori Ogata wrote:
>>>> Hi Aleskey,
>>>> Thank you for your comment.
>>>> I forgot to mention one thing.  I verified that all accesses to
>>>> langReflectioAccess calls the accessor "langReflectAccess()".  
>>>> Since this
>>>> variable is modified once from null to non-null, I think the write 
>>>> in the
>>>> setter is guaranteed to be visible in the getter by putting the
>>>> synchronized block in langReflectAccess().
>>>> Should I put comments about this assumption?  Actually, in JDK11 
>>>> there are
>>>> some lines that do not call the getter, so backport needs to fix them,
>>>> too.
>>>> Regards,
>>>> Ogata
>>>> Aleksey Shipilev <shade at redhat.com> wrote on 2019/07/17 16:49:08:
>>>>> From: Aleksey Shipilev <shade at redhat.com>
>>>>> To: Kazunori Ogata <OGATAK at jp.ibm.com>, 
>>>>> core-libs-dev at openjdk.java.net
>>>>> Date: 2019/07/17 16:49
>>>>> Subject: [EXTERNAL] Re: RFR: JDK-8227831: Avoid using volatile for
>>>> write-
>>>>> once, read-many class field
>>>>> On 7/17/19 8:48 AM, Kazunori Ogata wrote:
>>>>>> Webrev: http://cr.openjdk.java.net/~ogatak/8227831/webrev/
>>>>> Note this is generally not safe: it introduces data race on
>>>>> langReflectAccess field access. It has
>>>>> to be proved that everything that implements LangReflectAccess 
>>>>> interface
>>>>> makes this data race
>>>>> benign: e.g. all fields that are accessed via those implementation 
>>>>> are
>>>>> final, read once, etc. And
>>>>> briefly looking at that, I am not sure it is the case for the actual
>>>>> accessor generators.
>>>>> I'd cautiously leave "volatile" here.
>>>>> -- 
>>>>> Thanks,
>>>>> -Aleksey
>>>>> [attachment "signature.asc" deleted by Kazunori Ogata/Japan/IBM]

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