Re: NotifyFramePop and FramePop events

Tue, 05 Sep 2017 23:06:07 -0700

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 <jcbey...@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(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(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

            

          

        

        

      

    

    

  























					Reply via email to