Is it possible to stop a specific application thread?

[Keith McGuigan]

I had a prototype at one point that did #2, but I don't have the source for
it anymore. If Karen or Coleen kept a copy of my workspace, perhaps they
can reply with the diffs to give someone a leg up in doing this again.

--
- Keith (McGuigan)

On Monday, November 11, 2013, David Holmes wrote:

> On 12/11/2013 11:16 AM, Keith Chapman wrote:
>
>> Hi David,
>>
>> On Mon, Nov 11, 2013 at 7:26 PM, David Holmes <david.holmes at oracle.com
>> <mailto:david.holmes at oracle.com>> wrote:
>>
>>     Hi Keith,
>>
>>
>>     On 12/11/2013 6:29 AM, Keith Chapman wrote:
>>
>>         Hi,
>>
>>         I'm playing around with some research ideas on hotspot.
>>
>>         One of my runtime services of the VM needs to stop a specific
>>         application
>>         thread (I have a handle to the pthread of the application
>>         thread). How can
>>         the VM stop such a thread? Is there any mechanism in place to do
>>         this?
>>
>>
>>     There is no general purpose mechanism currently in place.
>>
>>
>> Thats a bummer, having such a mechanism would have been useful for
>> numerous VM services like biased locking revocation, concurrent GC.
>>
>
> For which safepoints are generally used so a per-thread safepoint
> mechanism is what you are looking for.
>
>
>>     You could use the deprecated and inherently dangerous Thread.suspend
>>     mechanism (either directly or via the JVMTI suspension interface).
>>
>>     Or if you are more adventurous with your VM hacking you could
>>     introduce a per-thread safepoint operation to cause it to stop.
>>
>>
>> I actually thought about that and explored the way safepoints are
>> implemented. The place that I had most trouble was getting threads
>> running compiled code to stop. The way it currently works is by
>> allocating a polling page (at startup) and drilling in that address into
>> the compiled code. Hence i I am to do it per thread I would have to make
>> a call to the runtime to get the address of the polling page for that
>> particular thread. I presume this would be too expensive (Making a VM
>> call at the end of each method and through some loop iterations).
>>
>> Is there any insight you could offer on how this could be done?
>>
>
> Two basic options I can think of:
>
> 1. Use a single shared page and have other threads simply ignore the
> safepoint request. This might incur some overhead as not-wanted threads
> could hit this multiple times.
>
> 2. Use a per-thread page accessed via the current thread. There would be a
> little overhead for the indirection but the compiler knows how to access
> the current JavaThread object. No need for runtime calls that I can see.
>
> Good luck.
>
> David
>
>  Thanks,
>> Keith.
>>
>>     Or you could hack in a special "magic exception" type the processing
>>     of which simply blocks the thread until "resumed" somehow (not sure
>>     how ??). Or ...
>>
>>     Cheers,
>>     David
>>
>>         Thanks,
>>         Keith.
>>
>>
>>
>>
>> --
>>
>> Keith Chapman
>> blog: http://www.keith-chapman.org
>>
>