NotifyFramePop and FramePop events

Wed Sep 6 17:29:28 UTC 2017

That is only cleared if the state is enabled for JVMTI_EVENT_FRAME_POP. If
you disable it before, you'd skip that.

When I go look at the jvmtiEventController and peruse, it seems that if you
can have the case I am talking about if all the threads turn off the
JVMTI_EVENT_FRAME_POP, then the state would be off as well and then we
would not clear (or even check for that matter).

Am I wrong?

On Tue, Sep 5, 2017 at 11:05 PM, serguei.spitsyn at oracle.com <
serguei.spitsyn at oracle.com> wrote:

> Hi JC,
>
> Thank you for verifying this!
>
>
>
> On 9/5/17 11:01, JC Beyler wrote:
>
> Hi all,
>
> After looking at the code a bit more, I don't see a viable way of really
> either:
>    - At notification disabling, remove all pop event requests
>    - Enforcing that the current frame at depth is singled out, if you
> disable and then pop that frame, the event is destroyed with the frame so
> that a subsequent pop at that depth no longer fires
>
> Because of that, I'd recommend just changing the documentation to
> highlight this, it might look a bit like this:
>
> --- old/src/share/vm/prims/jvmti.xml	2017-09-05 10:51:05.850810934 -0700
> +++ new/src/share/vm/prims/jvmti.xml	2017-09-05 10:51:05.710811367 -0700
> @@ -2907,7 +2907,7 @@
>      <function id="NotifyFramePop" num="20">
>        <synopsis>Notify Frame Pop</synopsis>
>        <description>
> -	When the frame that is currently at <paramlink id="depth"></paramlink>
> +	When a frame currently at <paramlink id="depth"></paramlink>
>          is popped from the stack, generate a
>  	<eventlink id="FramePop"></eventlink> event.  See the
>  	<eventlink id="FramePop"></eventlink> event for details.
> @@ -2916,6 +2916,12 @@
>          <p/>
>          The specified thread must either be the current thread
>          or the thread must be suspended.
> +        <p/>
> +        Note: if the notification event is disabled and a frame at
> +        <paramlink id="depth"></paramlink> is popped, no event is generated.
> +        After re-enabling the notification event, the registered
> +        <paramlink id="depth"></paramlink> is still active and will provoke an
> +        event when a frame at <paramlink id="depth"></paramlink> is popped.
>        </description>
>        <origin>jvmdi</origin>
>        <capabilities>
>
> Would someone want to review this and tell me if I should create a webrev for it? Or tell me hell no ;-)
>
>
> I'd say, "hell no" as it looks wrong to me. ;-)
> Please, see a comment below.
>
>
> Thanks!
>
> Jc
>
>
> On Thu, Aug 31, 2017 at 3:15 PM, JC Beyler <jcbeyler at google.com> wrote:
>
>> Hi all,
>>
>> I was asked to raise a question about NotifyFramePop and FramePop events
>> and I thought I would just ask it here:
>>
>> If we imagine we have a stack frame such as:
>>
>> call_depth0
>>   call_depth1
>>     call_depth2
>>        call_depth3
>>
>> And at this third depth, we request a frame pop when leaving depth1 via
>> the NotifyFramePop call. We would of course assume that when leaving
>> call_depth1 we get a FramePop event.
>>
>> Now imagine that we disable the frame pop event notification in
>> call_depth2:
>> call_depth0
>>   call_depth1
>>     call_depth2
>>        SetEventNotificationMode
>> <https://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#SetEventNotificationMode>
>> (JVMTI_DISABLE, JVMTI_EVENT_FRAME_POP, NULL)
>>
>> If the stack now pops back to call_depth0, the frame pop system is not
>> checked, the FramePop for call_depth1 is not issued either.
>>
>> However, imagine now that later down the road, the stack trace has built
>> itself back up and we enabled the event:
>> call_depth0
>>   second_call_depth1
>>     second_call_depth2
>>        SetEventNotificationMode
>> <https://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#SetEventNotificationMode>
>> (JVMTI_ENABLE, JVMTI_EVENT_FRAME_POP, NULL)
>>
>> Then when leaving second_call_depth1, we seemingly will issue a frame pop
>> event.
>>
>
> No.
> The NotifyFramePop request has been already cleaned at the first
> call_depth0 pop.
> Please, see the following line in the jvmtiExport.cpp:
> JvmtiExport::post_method_exit():
>           ets->clear_frame_pop(cur_frame_number);
>
> We have to make another NotifyFramePop request to get this one.
>
> Thanks,
> Serguei
>
>
>
>> Here is the qualm:
>>   - It seems counter intuitive and the documentation does not
>> specify/warn about this; it seems that if you disable the events you should
>> perhaps clear up the pop requests.
>>   - At least the documentation for NotifyFramePop (
>> https://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.
>> html#NotifyFramePop) should be changed since it says: "When the frame
>> that is currently at depth is popped from the stack" to something like
>> "When the first frame at the depth is popped from the stack and the event
>> notification is enabled"
>>
>> Therefore the questions are:
>>
>> 1) Is this such a corner case, that we do not wish to try to fix the
>> documentation or the code?
>> 2) Is this a corner case we do not wish to handle, therefore put a fix in
>> the documentation to at least warn users of this
>> 3) Is this a bug that we'd like to fix?
>>
>> Thanks for your insight,
>> Jc
>>
>
>
>
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