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 <sh...@redhat.com> wrote on 2019/07/17 16:49:08:
From: Aleksey Shipilev <sh...@redhat.com>
To: Kazunori Ogata <oga...@jp.ibm.com>, core-libs-dev@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]
