Hi David,
"This function will be called by the VM when the library is about to be unloaded.
The library will be unloaded (unless it is statically linked into the executable) and
this function will be called if some platform specific mechanism causes the unload (an
unload mechanism is not specified in this document) or the library is (in effect)
unloaded by the termination of the VM. VM termination includes normal termination and VM
failure, including start-up failure, but not, of course, uncontrolled shutdown. An
implementation may also choose to not call this function if the
Agent_OnAttach/Agent_OnAttach_L function reported an error (returned a non-zero
value)."
Thank you so much!
According to this, we can fix this bug simply. (We can unload agent library
without `Agent_OnUnload()` call)
However I think following sentence should be removed because DLL unloading
mechanism is not hooked, and also `Agent_OnUnload()` wouldn't be called by dll
unloading - it depends on VM termination / failure / non-zero value from entry
points.
"this function will be called if some platform specific mechanism causes the unload
(an unload mechanism is not specified in this document)"
If you are OK, I will update CSR.
Cheers,
Yasumasa
On 2020/12/16 11:09, David Holmes wrote:
Hi Yasumasa,
Sorry for the delay getting back to this.
On 1/12/2020 4:41 pm, Yasumasa Suenaga wrote:
Hi David,
On 2020/12/01 14:59, David Holmes wrote:
Looking at the original webrev from September:
https://cr.openjdk.java.net/~ysuenaga/JDK-8252657/webrev.00/src/hotspot/share/prims/jvmtiExport.cpp.cdiff.html
The suggestion to unload the library if Agent_OnAttach fails seems quite
reasonable - it is what happens if no Agent_OnAttach function can be found in
the agent.
But the specification is lacking because it simply states any such error is "ignored" -
which is an oversimplification and I think was really intended to contrast with the abort behaviour
if Agent_OnLoad fails. In addition the specification indicates that Agent_OnUnload is called any
time the agent library is unloaded, except for "uncontrolled shutdown". This
unfortunately suggests that before unloading the library after OnAttach fails, we also call
OnUnload. That seems wrong and in fact the VM does not do that - and noone seems to have complained.
Also note the specification states an agent "must export a start-up function
..." but doesn't say what happens if it doesn't do so.
My gut feeling for what the specification should say here is that if the
start-up function does not exist, or the call to Agent_OnAttach reports an
error, then the agent library is immediately unloaded with no attempt to call
the agent shutdown function (Agent_OnUnload).
That's what I wanted to suggest!
I understand we need to consider following case in this issue, is it right?
Agent_OnLoad / Agent_OnLoad_L does not exist:
- Agent_OnUnload is not called
- DLL is not unloaded (JVM will abort)
Agent_OnLoad / Agent_OnLoad_L failed:
- Agent_OnUnload is not called
- DLL is not unloaded (JVM will abort)
These cases are already well covered.
Agent_OnAttach does not exist:
- Agent_OnUnload is not called
- DLL is unloaded
Agent_OnAttach_L does not exist:
- Agent_OnUnload is not called
- DLL is not unloaded (static link)
These cases are kind of implicitly covered, but could be clarified.
Agent_OnAttach failed:
- Agent_OnUnload is not called
- DLL is unloaded
Agent_OnAttach_L faled:
- Agent_OnUnload is not called
- DLL is not unloaded (static link)
These are the problematic cases.
But with my "compatibility glasses" on this may be too strong to retrofit to
the specification as other VMs may behave differently. So as I stated in the CSR request
we probably want to allow the current hotspot behaviour but not mandate it (unless we
check with all the other VM implementations that such a specification is okay with them).
Ok, for example, can we change the spec as following?
(Of course, this is not all)
```
diff --git a/src/hotspot/share/prims/jvmti.xml
b/src/hotspot/share/prims/jvmti.xml
index 44553b8065f..57c87b1a71b 100644
--- a/src/hotspot/share/prims/jvmti.xml
+++ b/src/hotspot/share/prims/jvmti.xml
@@ -688,7 +688,8 @@ Agent_OnUnload_L(JavaVM *vm)</example>
mechanism causes the unload (an unload mechanism is not specified in this
document)
or the library is (in effect) unloaded by the termination of the VM
whether through
normal termination or VM failure, including start-up failure.
- Uncontrolled shutdown is, of course, an exception to this rule.
+ Uncontrolled shutdown is, of course, an exception to this rule, and also
it might not be called
+ when start-up function does not exist or fails (returns non-zero value).
I think this is what was originally proposed and as I said then I don't like
burying this detail inside the reference to uncontrolled shutdown. We should
make this much more explicit which requires some reworking of the existing
far-too-long sentence.
"This function will be called by the VM when the library is about to be unloaded.
The library will be unloaded (unless it is statically linked into the executable) and
this function will be called if some platform specific mechanism causes the unload (an
unload mechanism is not specified in this document) or the library is (in effect)
unloaded by the termination of the VM. VM termination includes normal termination and VM
failure, including start-up failure, but not, of course, uncontrolled shutdown. An
implementation may also choose to not call this function if the
Agent_OnAttach/Agent_OnAttach_L function reported an error (returned a non-zero
value)."
Cheers,
David
-----
Note the distinction between this function and the
<eventlink id="VMDeath">VM Death event</eventlink>: for the VM Death event
to be sent, the VM must have run at least to the point of initialization
and a valid
```
I agree that the more general issue of re-loading an agent is a separate issue.
Thanks!
Yasumasa
David
-----
On 1/12/2020 3:21 pm, David Holmes wrote:
On 1/12/2020 3:19 pm, David Holmes wrote:
On 1/12/2020 2:46 pm, Yasumasa Suenaga wrote:
Hi Chris, David,
Currently Agent_OnUnload() is not called when Agent_OnLoad() is failed - JVM
will abort.
Should we re-think this behavior?
We should we rethink that? It is probably one of the clearest parts of the
spec. If Agent_Onload fails that is considered a fatal error - end of story.
I meant, of course, "Why should we rethink that?".
David
The issue is with Agent_onAttach and how its failure should, or should not,
impact Agent_OnUnload.
David
-----
https://github.com/YaSuenag/jvmti-examples/tree/master/helloworld
```
$ java -agentpath:/path/to/libhelloworld.so=error --version
Hello World from Agent_OnLoad()
options = error
Error occurred during initialization of VM
agent library failed to init: /path/to/libhelloworld.so
```
Thanks,
Yasumasa
On 2020/12/01 11:44, David Holmes wrote:
On 1/12/2020 11:45 am, Chris Plummer wrote:
On Fri, 2 Oct 2020 07:27:43 GMT, Yasumasa Suenaga <ysuen...@openjdk.org> wrote:
* Q3: What has to be done for statically linked agent?
JVMTI spec says "unless it is statically linked into the executable", so I
think we can ignore about Agent_OnUnload_L() in this PR.
I don't think that makes sense. If you call it for dynamically linked then you
need to call it for statically linked also.
Agreed. Even though you can't physically unload the statically linked library,
if it is logically unloaded by some mechanism, then Agent_OnUnload_L is
supposed to be called.
David
-----
-------------
PR: https://git.openjdk.java.net/jdk/pull/19
