RFR(XXS): 8007147: Trace event ExecuteVMOperation may get dangling pointer

[Dean Long]

When the VM_Operation is created, we could take a snapshot of the 
thread_id of the caller, then use that later.
Or we could block the creating thread from fully exiting until the VM op 
executes.

dl

On 2/19/2013 1:59 AM, Staffan Larsen wrote:
> I don't like this solution since we are swallowing information that could potentially be interesting (that a VMOperation happened). I think it's fine to say that in the case of non-blocking VMOperations, the caller thread will be undefined or set to 0. We could add that information to the event field description in traceevents.xml so that it is documented.
>
> /Staffan
>
>
> On 19 feb 2013, at 10:47, Markus Grönlund <markus.gronlund at oracle.com> wrote:
>
>> Hi David,
>>
>> Thanks for taking a look.
>>
>> The original idea was to use thread id 0 as a signal for "Unknown Thread" or "no thread" if you like - however, there is nothing actually enforcing this; it was more of an implicit assumption (and to be honest, the only OS which I know for sure will never give a tid of 0 is Windows). I have tried to find information if a 0 will ever be given out by pthreads and mach_t, but I mostly find info on "process relative opaque id"...so it could be likely that a 0 is used as a valid tid here (probably unlikely but still...)
>>
>> Also, I see that the tracing output just happily gives out tid == 0 if I write it in as suggested in webrev01...I am not entirely happy with this. If we view 0 as "Unknown Thread", this should maybe be treated differently.
>>
>> I don't know yet what Id I will be able to use here to signal the fact that thread is unknown (in a platform agnostic way) - but I will try to come up with something.
>>
>> In the interim, we should just avoid committing the event for non-concurrent VM operations event altogether (I have mostly seen these as EnableBiasLocking and Thread.Stop's).
>>
>> The updated webrev02 gives a suggestion on only committing the tracing event for non-concurrent vm ops:
>>
>> http://cr.openjdk.java.net/~mgronlun/8007147/webrev02/
>>
>> (follow up work will also need rework the event fields).
>>
>> Thanks
>> Markus
>>
>>
>>
>> -----Original Message-----
>> From: David Holmes
>> Sent: den 18 februari 2013 05:06
>> To: Markus Grönlund
>> Cc: serviceability-dev at openjdk.java.net; hotspot-runtime-dev at openjdk.java.net
>> Subject: Re: RFR(XXS): 8007147: Trace event ExecuteVMOperation may get dangling pointer
>>
>> Hi Markus,
>>
>> So is zero as a thread id defined to mean "no thread" ?
>>
>> David
>>
>> On 13/02/2013 12:03 AM, Markus Grönlund wrote:
>>> Greetings,
>>>
>>> Kindly asking for reviews and a putback sponsorship for the following
>>> change:
>>>
>>> Bugid: http://bugs.sun.com/view_bug.do?bug_id=8007147
>>>
>>> Webrev: http://cr.openjdk.java.net/~mgronlun/8007147/webrev01/
>>>
>>> Please also note this is for hs24.
>>>
>>> Thanks to David Holmes for pointing out this problem.
>>>
>>> Thanks
>>>
>>> Markus
>>>