Hi David,
Thanks for the review. Updated webrev:
http://cr.openjdk.java.net/~iklam/jdk12/8212200-cds-jvmti-clfh-critical-classes.v04/
http://cr.openjdk.java.net/~iklam/jdk12/8212200-cds-jvmti-clfh-critical-classes.v04.delta/
More comments below:
On 10/21/18 6:57 PM, David Holmes wrote:
> Hi Ioi,
>
> Generally seems okay.
>
> On 22/10/2018 11:15 AM, Ioi Lam wrote:
>> Re-sending to the correct mailing lists. Please disregard the other
>> email.
>>
>> http://cr.openjdk.java.net/~iklam/jdk12/8212200-cds-jvmti-clfh-critical-classes.v03/
>>
>> https://bugs.openjdk.java.net/browse/JDK-8212200
>>
>> Hi,
>>
>> CDS has various built-in assumptions that classes loaded by
>> SystemDictionary::resolve_well_known_classes must not be replaced
>> by JVMTI ClassFileLoadHook during run time, including
>>
>> - field offsets computed in JavaClasses::compute_offsets
>> - the layout of the strings objects in the shared strings table
>>
>> The "well-known" classes can be replaced by ClassFileLoadHook only
>> when JvmtiExport::early_class_hook_env() is true. Therefore, the
>> fix is to disable CDS under this condition.
>
> I'm a little unclear why we have to iterate JvmtiEnv list when this
> has to be checked during JVMTI_PHASE_PRIMORDIAL?
>
I think you are asking about this new function? I don't like the name
"early_class_hook_env()". Maybe I should change it to
"has_early_class_hook_env()"?
bool JvmtiExport::early_class_hook_env() {
JvmtiEnvIterator it;
for (JvmtiEnv* env = it.first(); env != NULL; env = it.next(env)) {
if (env->early_class_hook_env()) {
return true;
}
}
return false;
}
This function matches condition in the existing code that would call
into ClassFileLoadHook:
class JvmtiClassFileLoadHookPoster {
...
void post_all_envs() {
JvmtiEnvIterator it;
for (JvmtiEnv* env = it.first(); env != NULL; env = it.next(env)) {
..
post_to_env(env, true);
}
}
...
void post_to_env(JvmtiEnv* env, bool caching_needed) {
if (env->phase() == JVMTI_PHASE_PRIMORDIAL &&
!env->early_class_hook_env()) {
return;
}
post_all_envs() is called just before a class is about to be loaded in
the JVM. So if *any* env->early_class_hook_env() returns true, there's a
chance that it will replace a well-known class.So, as a preventive
measure, CDS will be disabled.
>> I have added a few test cases to try to replace shared classes,
>> including well-known classes and other classes. See
>> comments in ReplaceCriticalClasses.java for details.
>>
>> As a clean up, I also renamed all use of "preloaded" in
>> the source code to "well-known". They refer to the same thing
>> in HotSpot, so there's no need to use 2 terms. Also, The word
>> "preloaded" is ambiguous -- it's unclear when "preloading" happens,
>> and could be confused with what CDS does during archive dump time.
>
> A few specific comments:
>
> src/hotspot/share/classfile/systemDictionary.cpp
>
> + bool SystemDictionary::is_well_known_klass(Symbol* class_name) {
> + for (int i = 0; ; i++) {
> + int sid = wk_init_info[i];
> + if (sid == 0) {
> + break;
> + }
>
> I take it a zero value is a guaranteed end-of-list sentinel?
>
Yes. The array is defined just a few lines above:
static const short wk_init_info[] = {
#define WK_KLASS_INIT_INFO(name, symbol) \
((short)vmSymbols::VM_SYMBOL_ENUM_NAME(symbol)),
WK_KLASSES_DO(WK_KLASS_INIT_INFO)
#undef WK_KLASS_INIT_INFO
0
};
Also,
class vmSymbols: AllStatic {
enum SID {
NO_SID = 0,
....
> + for (int i=FIRST_WKID; i<last; i++) {
>
> Style nit: need spaces around = and <
>
Fixed.
> ---
>
> test/hotspot/jtreg/runtime/SharedArchiveFile/serviceability/ReplaceCriticalClasses.java
>
>
> New file should have current copyright year only.
>
Fixed.
> 31 * @comment CDS should not be disabled -- these critical classes
> will be replaced because JvmtiExport::early_class_hook_env() is true.
>
> Comment seems contradictory: if we replace critical classes then CDS
> should be disabled right??
>
Fixed.
> I expected to see tests that checked for:
>
> "CDS is disabled because early JVMTI ClassFileLoadHook is in use."
>
> in the output. ??
>
<rant>
It would have been easy if jtreg lets you check the output of @run
easily. Instead, your innocent suggestion has turned into 150+ lines of
new code :-( Maybe "let's write all shell tests in Java" isn't such a
great idea after all.
</rant>
Now the test checks that whether CDS is indeed disabled, whether the
affected class is loaded from the shared archive, etc.
Thanks
- Ioi
> Thanks,
> David
>
>
>>
>> > In early e-mails Jiangli wrote:
>> >
>> > We should consider including more classes from the default classlist
>> > in the test. Archived classes loaded during both 'early' phase and
>> after
>> > should be tested.
>>
>> Done.
>>
>>
>> > For future optimizations, we might want to prevent loading additional
>> > shared classes if any of the archived system classes is changed.
>>
>> What's the benefit of doing this? Today we already stop loading a shared
>> class if its super class was not loaded from the archive.
>>
>>
>> Thanks
>> - Ioi
>>