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

[Markus Grönlund]

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
>