RFR: 8343938: TestStressBailout triggers "Should not be locked when freed" assert [v2]

[Theo Weidmann]

> This patch addresses a race condition that occurs when the compiler queues shut down because compilation was disabled forever.
> 
> The following changes enforce a scheduling such that the race condition (almost) always occurs and the JVM crashes:
> 
> 
> diff --git a/src/hotspot/share/compiler/compileBroker.cpp b/src/hotspot/share/compiler/compileBroker.cpp
> index 518a53ca26e..61434d1f5f2 100644
> --- a/src/hotspot/share/compiler/compileBroker.cpp
> +++ b/src/hotspot/share/compiler/compileBroker.cpp
> @@ -371,6 +371,10 @@ void CompileQueue::free_all() {
>        MutexLocker ct_lock(current->lock());
>        current->lock()->notify();
>      }
> +    do {
> +      sleep(2);
> +    } while (current->is_free());
> +    tty->print_cr("!! other thread has the lock! let's be fast and call free to crash!");
>      // Put the task back on the freelist.
>      CompileTask::free(current);
>    }
> @@ -1739,6 +1743,8 @@ void CompileBroker::wait_for_completion(CompileTask* task) {
>      free_task = true;
>      while (!task->is_complete() && !is_compilation_disabled_forever()) {
>        ml.wait();
> +      tty->print_cr("I woke up and will give the other thread 10 seconds to crash!");
> +      sleep(10);
>      }
>    }
> 
> 
> The reason for the race condition is that in `CompileQueue::free_all()` the wake up (`notify`, line 372 below) to the thread waiting for the task is immediately succeeded by calling `CompileTask::free` on the task:
> 
> https://github.com/openjdk/jdk/blob/4b4b1e912a3193cc95c956acc770015f707449b1/src/hotspot/share/compiler/compileBroker.cpp#L361-L375
> 
> `CompileTask::free` asserts that the task is not locked:
> 
> https://github.com/openjdk/jdk/blob/4b4b1e912a3193cc95c956acc770015f707449b1/src/hotspot/share/compiler/compileTask.cpp#L65-L69
> 
> However, as shown by the diff above, a scheduling exists, where the waiting thread in `CompileBroker::wait_for_completion` will acquire the lock as part of the wake up (line 1741 below) *before* the other thread calls `CompileTask::free` but *not make any progress* (that is, keeps the lock) before the other thread executes `CompileTask::free`. In this case, the assert triggers as the lock is locked.
> 
> https://github.com/openjdk/jdk/blob/4b4b1e912a3193cc95c956acc770015f707449b1/src/hotspot/share/compiler/compileBroker.cpp#L1737-L1749
> 
> The patch adds a counter to each task to indicate if other threads are waiting for the task. `CompileQueue::free_all` wakes up all waiting threads before continuing and only tries to free the task itself if no other threads were waiting.

Theo Weidmann has updated the pull request incrementally with one additional commit since the last revision:

  Add asserts

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23195/files
  - new: https://git.openjdk.org/jdk/pull/23195/files/4df7eadd..79237e60

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23195&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23195&range=00-01

  Stats: 9 lines in 1 file changed: 7 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23195.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23195/head:pull/23195

PR: https://git.openjdk.org/jdk/pull/23195