RFR(XXS): 8007147: Trace event ExecuteVMOperation may get dangling pointer
Markus Grönlund
markus.gronlund at oracle.com
Wed Feb 20 02:15:17 PST 2013
Thanks for your input on this one.
I think I am hearing that we can proceed with assigning a 0 (zero) as the thread id to indicate the thread information is unknown for a non-blocking operation. Risk being of course to have a false "hit" if a 0 is assigned by some OS as a thread id, and even worse if 0 is also re-used across threads. This should be considered low risk. In addition, the occasional wrong info in the caller thread field might not warrant avoiding presenting info about non-blocking operations.
Resolving this would incorporate a lot of investigation which must be dealt with outside the scope of this bug.
By adding additional comments about this fact (thread 0 being used to indicate "thread unknown" for non-blocking ops) I think we can proceed with a modified version of the first webrev01, but with additional comments added.
Updated webrev can be found here:
http://cr.openjdk.java.net/~mgronlun/8007147/webrev03/
Thanks again
Markus
-----Original Message-----
From: David Holmes
Sent: den 20 februari 2013 03:38
To: Dean Long
Cc: Staffan Larsen; Markus Grönlund; 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
On 20/02/2013 5:30 AM, Dean Long wrote:
> On 2/19/2013 11:00 AM, Staffan Larsen wrote:
>>
>> On 19 feb 2013, at 19:56, Dean Long <dean.long at oracle.com
>> <mailto:dean.long at oracle.com>> wrote:
>>
>>> When the VM_Operation is created, we could take a snapshot of the
>>> thread_id of the caller, then use that later.
>>
>> One problem with that is if the thread_id gets reused for a new
>> thread quickly after the old thread terminates. This is perhps
>> ulikely, but could happen.
>>
>>> Or we could block the creating thread from fully exiting until the
>>> VM op executes.
>>
>> That would introduce a lot more synchronization and state to keep
>> track of.
>>
> I think we could do it with a reference count on the thread, a wait in
> thread exit, and a notify in the VM thread.
> We have a similar dangling pointer problem with JVMTI (7154963), and a
> reference count should solve that problem as well.
I think that proposal needs a lot more investigation, certainly it is well out of scope for this bug. There are potential dangling thread pointers all over the JVM interfaces.
David
-----
>
> dl
>
>> /Staffan
>>
>
More information about the serviceability-dev
mailing list