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

[Claes Redestad]

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]
>>>