RFR: Fix another deadlock with oom_during_evacuation()

[Aleksey Shipilev]

On 01/11/2017 12:35 PM, Roman Kennke wrote:
> I encounter another deadlock involving oom_during_evacuation():
> 
> 1. One (or more) Java threads get into oom_during_evacuation(), waiting
> for _evacuation_in_progress to become false.
> 2. Some other thread(s) tries to execute a VM task (non-Shenandoah).
> The safepoint-begin protocol acquires the Threads_lock.
> 3. The ShenandoahConcurrentThread tries to turn off evacuation, and
> this attempts to acquire the Threads_lock too, but can't because of 2.
> 2 is waiting for 1 to get to a safepoint. 1 cannot get there as long as
> 3 hasn't turned off evacuation. -> deadlock
> 
> My solution is to set the _evacuation_in_progress flag to false without
> the Threads_lock. Threads_lock is only required when turning off the
> thread-local flag. This allows 1 to proceed and get to a safepoint, and
> thus resolve the deadlock.
> 
> http://cr.openjdk.java.net/~rkennke/fixoomdeadlock/webrev.00/
> 
> Ok to push? I would also like to push this to jdk8 right away.

Ok.

Thanks,
-Aleksey