RFR: 8298377: JfrVframeStream causes deadlocks in ZGC [v2]

[Erik Österlund]

On Wed, 18 Jan 2023 13:00:46 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

> > > > > Question: does the "start_processing()" call ever reach a malloc, directly or indirectly? If so, it is problematic to call from the sampler thread.
> > > > 
> > > > 
> > > > It does allocate a stack iterator with malloc if it really did need to start processing and it was not yet started. So yes. Would you mind explaining why using malloc is problematic?
> > > 
> > > 
> > > The problem is that the suspendee can be holding the malloc lock. Please see https://bugs.openjdk.org/browse/JDK-8274298 for an example situation.
> > 
> > 
> > Right - that makes sense. But I thought the processing is poked here before we suspend the target thread? Or did I read the code wrong?
> 
> Ah. The call is now located before the suspend is attempted. That is good from a sampling perspective. Is it mt safe to call start_processing() on another running thread?

Yes, it is perfectly thread safe, because it has a per-thread lock that ensures mutual exclusion. Other than that, the fact that we also hold the Threads_lock here means that we can guarantee it remains processed until the signal hits the target thread.

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PR: https://git.openjdk.org/jdk/pull/11586