Hi Folks,

It is with great pleasure that I can declare that a new website is on its way.

It is currently hosted at https://openmama.github.io as a preview version (the former documentation website address) and therefore I invite the members of the mailing list to have a look, and report any issues back directly to be before its "proper" launch to openmama.org on Monday 28th September.

The site is a complete overhaul and has several key improvements over the former site beyond aesthetics:

• It is responsive
• It is a static, bloat-free website so it should load very quickly
• You can submit your own suggested changes to the site via pull request because it's all on github
• It merges the documentation and main marketing website into one
• It generally tries to de-duplicate information and the content has had a general refresh

Anyway if there are any issues please raise them at https://github.com/OpenMAMA/openmama.github.io/issues and I hope you enjoy the new site! As usual, I’ll be keeping an eye on gitter if you have any immediate feedback / concerns.

Cheers,

Frank

Frank Quinn

Cascadium

T: +44 (0) 28 8678 8015

E: fquinn@...

W: http://cascadium.io

Sounds quite sensible to me, Frank.

Hi Folks,

I would like to propose that we change the OpenMAMA github default git branch to “next” instead of “master”.

This would mean that:

• When you visit our github page, you will see the latest development code and documentation
• When you clone the repository, you no longer need to change branch before making feature branches which will eventually become pull requests.

Note all existing clones etc would remain unchanged so this won’t actually “break” anything for existing OpenMAMA developers.

The rationale being that “master” is reflective of the last stable release, but it’s also the source of the main README output when visitors come to the github page which makes it look like the project is less active than it actually is, repo documentation effectively slows down to match the biannual release cycle etc.

The industry in general is also gradually moving away from the “master” nomenclature for branches with linux and github both making noises about moving away from defaulting to that branch name, so this would pave the way for eventual retirement of the branch name (which is actually fine - we can just use our release branches instead as we already do with minimal impact).

If anyone has any comments or feedback on this, please respond on this thread.

Cheers,

Frank

Frank Quinn

Cascadium

T: +44 (0) 28 8678 8015

E: fquinn@...

W: http://cascadium.io

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Hi Folks,

I would like to propose that we change the OpenMAMA github default git branch to “next” instead of “master”.

This would mean that:

• When you visit our github page, you will see the latest development code and documentation
• When you clone the repository, you no longer need to change branch before making feature branches which will eventually become pull requests.

Note all existing clones etc would remain unchanged so this won’t actually “break” anything for existing OpenMAMA developers.

The rationale being that “master” is reflective of the last stable release, but it’s also the source of the main README output when visitors come to the github page which makes it look like the project is less active than it actually is, repo documentation effectively slows down to match the biannual release cycle etc.

The industry in general is also gradually moving away from the “master” nomenclature for branches with linux and github both making noises about moving away from defaulting to that branch name, so this would pave the way for eventual retirement of the branch name (which is actually fine - we can just use our release branches instead as we already do with minimal impact).

If anyone has any comments or feedback on this, please respond on this thread.

Cheers,

Frank

Frank Quinn

Cascadium

T: +44 (0) 28 8678 8015

E: fquinn@...

W: http://cascadium.io

This message is confidential and subject to terms at: https://www.jpmorgan.com/emaildisclaimer including on confidential, privileged or legal entity information, malicious content and monitoring of electronic messages. If you are not the intended recipient, please delete this message and notify the sender immediately. Any unauthorized use is strictly prohibited.

Hi Folks,

I plan on taking a cut for OpenMAMA 6.3.1 RC1 over the coming weekend (21-Aug 2020). If anyone has anything they want to submit before then please let me know so I can delay the cut if necessary, otherwise I will continue as planned.

This release includes some changes to support:

• CentOS 8
• Fedora 31
• Fedora 32
• Ubuntu 20.04 LTS

It also effectively drops support for the now EOL:

• Fedora 28
• Fedora 29
• Ubuntu 14.04 LTS

This is in preparation for a RC that will be cut within the next few weeks which is now set up to include RPMs and Debian packages for these modern distros to make installation easy, take care of dependencies etc.

Cheers,

Frank

Frank Quinn

Cascadium

T: +44 (0) 28 8678 8015

E: fquinn@...

W: http://cascadium.io

Hi Mike,

Have raised https://github.com/OpenMAMA/OpenMAMA/issues/411 For follow up on this one – let’s track there for paper trail / release note reference etc.

Could you add some details on the execution environment? Particularly the shutdown sequence in the code, and whether or not this is JNI etc to see if anything unusual is compounding the issue?

The transport destroy is not supposed to be attempted by the app until after the subscriptions have all been destroyed and the queues have been drained and destroyed so it sounds like the bridge is still firing callbacks after the subscription has been “destroy”ed to free up the memory.

Cheers,

Frank

Hi OpenMAMA Dev,

We have encountered a deadlock situation in mamaSubscription’s and mamaTransport’s teardown logic due to lock ordering when destroying their underlying mamaPublisher. We are able to reliably reproduce this with the tick42 bridge. Could someone take a look at this for us please?

Thanks in advance,

Mike

Deadlock – note that subscription and transport attempt to destroy same publisher:
mamaTransport teardown:

1. transport.c:         mamaTransport_destroy()
2. transport.c:         mamaTransportImpl_clearTransportWithPublishers()
3. list.c:                    list_for_each() – Acquires list lock (1)
4. transport.c:         mamaTransportImpl_clearTransportPublisherCallback()
5. publisher.c:         mamaPublisherImpl_clearTransport()
6. publisher.c:         mamaPublisherImpl_destroy() – Attempts to acquire publisher lock (2)

mamaSubscription teardown

1. subscription.c:    mamaSubscriptionImpl_onSubscriptionDestroyed()
2. subscription.c:    mamaSubscription_cleanup()
3. publisher.c:         mamaPublisherImpl_destroy() – Acquires publisher lock (2)
4. transport.c:         mamaTransport_removePublisher()
5. list.c:                    list_remove_element() – Attempts to acquire list lock (1)

This message is confidential and subject to terms at: https://www.jpmorgan.com/emaildisclaimer including on confidential, privileged or legal entity information, viruses and monitoring of electronic messages. If you are not the intended recipient, please delete this message and notify the sender immediately. Any unauthorized use is strictly prohibited.

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Hi Mike – did you have any joy with testing this one?

Hi Mike,

I have put together something that looks like it address this (I modified qpid and MamaListen locally with various sleeps to verify behaviour). Turned out to be a little trickier than I had anticipated with the different paths through the state machine.

It works by assigning an atomic reference counter to increment for the bridge, and the Java wrapper, such that only when receiving a statement of disinterest from both will it release the memory. Since calling deallocate is an admission that you will no longer be using that memory any more, operating on a first past the post system should be safe.

There are many paths through the MAMA subscription state machine though so I'd appreciate if you could give it a test in your target environment before I merge it in. You can find my development branch for this change on:

https://github.com/fquinner/OpenMAMA/tree/feature/subscription-reference-count

Let me know if this works for you.

Cheers,

Frank

Thanks Frank.

If you could take a look at implementing this, that would be much appreciated.

Mike

OK thanks Mike I thought user interaction was the primary problem but sounds like it's the finalizers.

This is where the subscription stuff gets hairy because there are a few different but similar areas at play here.

You should ideally unlock before the mamaSubscriptionImpl_deallocate to avoid undefined behaviour in the destroy yes. However...

In Java, when the GC kicks in, it fires off the JNI Java_com_wombat_mama_MamaSubscription_deallocate method which in turn calls mamaSubscription_deallocate which does acquire the subscription's mCreateDestroyLock

which could be (already) held onto while mamaSubscriptionImpl_deallocate is then called (!). So we actually already have undefined behaviour on that path when you look at it so that's effectively the same bug.

The path with Java destroy goes JNI Java_com_wombat_mama_MamaSubscription_destroy which calls mamaSubscription_destroy which will usually deactivate the subscription, then go on to invoke the destroyed callback. I was suggesting at this point, we *do* hang onto the lock until that callback is completed to protect the subscription object.

The path mamaSubscriptionImpl_onSubscriptionDestroyed is a different beast when the middleware is letting MAMA know that a subscription has been destroyed which may be via mamaSubscription_destroy -> mamaSubscription -> deactivate. In this case, we currently unlock before the callback. I was suggesting this should be moved until after the callback, but before the deallocate. If we hung onto the lock until after the deallocate, then we'd just be emulating the buggy behaviour already present in mamaSubscription_deallocate.

But yes when I look this through there is a more subtle issue afoot - because we have onSubscriptionDestroyed which will be called here (already inside the lock)

destroy

  deactivate

    lock mCreateDestroyLock

    mamaSubscription_deactivate_internal

     bridge_destroy

        mamaSubscriptionImpl_onSubscriptionDestroyed

          lock mCreateDestroyLock

          unlock mCreateDestroyLock

          impldeallocate()

    unlock

  lock

Now, since wlocks are recursive and these are from the same thread, this won't deadlock and should already be protected from the finalizer, but it also may not happen in this order depending on how the ondestroy callback is implemented in the bridge, so you could get something more like this:

destroy

  deactivate

    lock mCreateDestroyLock

    mamaSubscription_deactivate_internal

      bridge_destroy

        ... bridge takes this under consideration ...

    unlock mCreateDestroyLock

  lock

... time passes...

mamaSubscriptionImpl_onSubscriptionDestroyed

  lock mCreateDestroyLock

  unlock mCreateDestroyLock

  impldeallocate() <-- this is where it looks like the GC has an opportunity to cause trouble?

If we are going to have multiple threads coming in like this, then yes I think an atomic reference counter in the subscription object to track when each resource depending on the subscription object has gained and lost interest would be preferable to holding onto the lock. Would fix the existing bug already in the finalizer too.

Is this something you're looking clarification on before implementing or you want me to have a look at implementing this?

Cheers,

Frank

Hi Frank,

Why do we need to unlock the mutex before the deallocate call? I understand that this call destroys the lock, but with the proposed wrapper around the lock the destroy call can defer the actual release of memory to the unlock call.

The early release could be exploited because the MamaSubscription Java wrapper calls deallocate in its finalize method. Due to this, the GC thread could acquire the lock and release memory in mamaSubscriptionImpl_deallocate while the lock-releasing thread is trying to use this memory to invoke the user callback.

Thanks,

Mike

Hi Mike,

Could you shed some more light on what exactly the problem is here? I have had a refresher in that area, and from what I can see, I can’t think of any reason to unlock that particular mutex before the user callback so I’m not opposed to moving until after that callback if that will resolve whatever issue you’re seeing? It will need to be before the deallocate though.

The only reason I could think of for why this was done in the first place is to avoid deadlock if the user calls something like mamaSubscription_deallocate or something else that uses that mutex from the callback, but then I guess they’d learn pretty quickly if they had fallen foul of that. Then I thought this might be somehow exploited in one of the Java or .NET wrappers but couldn’t find any evidence of that either. Plus that callback is more informative than anything else for the application, so if they are attempting to deallocate, and that’s causing funny business (or something like that?), we just need to make it clear in the callback documentation that they shouldn’t be doing that.

Cheers,

Frank

At first glance, that sounds like just “kicking the can down the road” — i.e., you’re still left with the problem of what to do with these wrappers such that you can tear them down in a thread-safe manner.

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This message is confidential and subject to terms at: https://www.jpmorgan.com/emaildisclaimer including on confidential, privileged or legal entity information, viruses and monitoring of electronic messages. If you are not the intended recipient, please delete this message and notify the sender immediately. Any unauthorized use is strictly prohibited.

This message is confidential and subject to terms at: https://www.jpmorgan.com/emaildisclaimer including on confidential, privileged or legal entity information, viruses and monitoring of electronic messages. If you are not the intended recipient, please delete this message and notify the sender immediately. Any unauthorized use is strictly prohibited.

Hi Mike,

I have put together something that looks like it address this (I modified qpid and MamaListen locally with various sleeps to verify behaviour). Turned out to be a little trickier than I had anticipated with the different paths through the state machine.

It works by assigning an atomic reference counter to increment for the bridge, and the Java wrapper, such that only when receiving a statement of disinterest from both will it release the memory. Since calling deallocate is an admission that you will no longer be using that memory any more, operating on a first past the post system should be safe.

There are many paths through the MAMA subscription state machine though so I'd appreciate if you could give it a test in your target environment before I merge it in. You can find my development branch for this change on:

https://github.com/fquinner/OpenMAMA/tree/feature/subscription-reference-count

Let me know if this works for you.

Cheers,

Frank

Thanks Frank.

If you could take a look at implementing this, that would be much appreciated.

Mike

OK thanks Mike I thought user interaction was the primary problem but sounds like it's the finalizers.

This is where the subscription stuff gets hairy because there are a few different but similar areas at play here.

You should ideally unlock before the mamaSubscriptionImpl_deallocate to avoid undefined behaviour in the destroy yes. However...

In Java, when the GC kicks in, it fires off the JNI Java_com_wombat_mama_MamaSubscription_deallocate method which in turn calls mamaSubscription_deallocate which does acquire the subscription's mCreateDestroyLock

which could be (already) held onto while mamaSubscriptionImpl_deallocate is then called (!). So we actually already have undefined behaviour on that path when you look at it so that's effectively the same bug.

The path with Java destroy goes JNI Java_com_wombat_mama_MamaSubscription_destroy which calls mamaSubscription_destroy which will usually deactivate the subscription, then go on to invoke the destroyed callback. I was suggesting at this point, we *do* hang onto the lock until that callback is completed to protect the subscription object.

The path mamaSubscriptionImpl_onSubscriptionDestroyed is a different beast when the middleware is letting MAMA know that a subscription has been destroyed which may be via mamaSubscription_destroy -> mamaSubscription -> deactivate. In this case, we currently unlock before the callback. I was suggesting this should be moved until after the callback, but before the deallocate. If we hung onto the lock until after the deallocate, then we'd just be emulating the buggy behaviour already present in mamaSubscription_deallocate.

But yes when I look this through there is a more subtle issue afoot - because we have onSubscriptionDestroyed which will be called here (already inside the lock)

destroy

  deactivate

    lock mCreateDestroyLock

    mamaSubscription_deactivate_internal

     bridge_destroy

        mamaSubscriptionImpl_onSubscriptionDestroyed

          lock mCreateDestroyLock

          unlock mCreateDestroyLock

          impldeallocate()

    unlock

  lock

Now, since wlocks are recursive and these are from the same thread, this won't deadlock and should already be protected from the finalizer, but it also may not happen in this order depending on how the ondestroy callback is implemented in the bridge, so you could get something more like this:

destroy

  deactivate

    lock mCreateDestroyLock

    mamaSubscription_deactivate_internal

      bridge_destroy

        ... bridge takes this under consideration ...

    unlock mCreateDestroyLock

  lock

... time passes...

mamaSubscriptionImpl_onSubscriptionDestroyed

  lock mCreateDestroyLock

  unlock mCreateDestroyLock

  impldeallocate() <-- this is where it looks like the GC has an opportunity to cause trouble?

If we are going to have multiple threads coming in like this, then yes I think an atomic reference counter in the subscription object to track when each resource depending on the subscription object has gained and lost interest would be preferable to holding onto the lock. Would fix the existing bug already in the finalizer too.

Is this something you're looking clarification on before implementing or you want me to have a look at implementing this?

Cheers,

Frank

Hi Frank,

Why do we need to unlock the mutex before the deallocate call? I understand that this call destroys the lock, but with the proposed wrapper around the lock the destroy call can defer the actual release of memory to the unlock call.

The early release could be exploited because the MamaSubscription Java wrapper calls deallocate in its finalize method. Due to this, the GC thread could acquire the lock and release memory in mamaSubscriptionImpl_deallocate while the lock-releasing thread is trying to use this memory to invoke the user callback.

Thanks,

Mike

Hi Mike,

Could you shed some more light on what exactly the problem is here? I have had a refresher in that area, and from what I can see, I can’t think of any reason to unlock that particular mutex before the user callback so I’m not opposed to moving until after that callback if that will resolve whatever issue you’re seeing? It will need to be before the deallocate though.

The only reason I could think of for why this was done in the first place is to avoid deadlock if the user calls something like mamaSubscription_deallocate or something else that uses that mutex from the callback, but then I guess they’d learn pretty quickly if they had fallen foul of that. Then I thought this might be somehow exploited in one of the Java or .NET wrappers but couldn’t find any evidence of that either. Plus that callback is more informative than anything else for the application, so if they are attempting to deallocate, and that’s causing funny business (or something like that?), we just need to make it clear in the callback documentation that they shouldn’t be doing that.

Cheers,

Frank

At first glance, that sounds like just “kicking the can down the road” — i.e., you’re still left with the problem of what to do with these wrappers such that you can tear them down in a thread-safe manner.

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Hi OpenMAMA Dev,

We have encountered a deadlock situation in mamaSubscription’s and mamaTransport’s teardown logic due to lock ordering when destroying their underlying mamaPublisher. We are able to reliably reproduce this with the tick42 bridge. Could someone take a look at this for us please?

Thanks in advance,

Mike

Deadlock – note that subscription and transport attempt to destroy same publisher:
mamaTransport teardown:

1. transport.c:         mamaTransport_destroy()
2. transport.c:         mamaTransportImpl_clearTransportWithPublishers()
3. list.c:                    list_for_each() – Acquires list lock (1)
4. transport.c:         mamaTransportImpl_clearTransportPublisherCallback()
5. publisher.c:         mamaPublisherImpl_clearTransport()
6. publisher.c:         mamaPublisherImpl_destroy() – Attempts to acquire publisher lock (2)

mamaSubscription teardown

1. subscription.c:    mamaSubscriptionImpl_onSubscriptionDestroyed()
2. subscription.c:    mamaSubscription_cleanup()
3. publisher.c:         mamaPublisherImpl_destroy() – Acquires publisher lock (2)
4. transport.c:         mamaTransport_removePublisher()
5. list.c:                    list_remove_element() – Attempts to acquire list lock (1)

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