RFR: JDK-8317661: [REDO] store/load order not preserved when handling memory pool due to weakly ordered memory architecture of aarch64

[Andrew Haley]

On Wed, 11 Oct 2023 09:56:37 GMT, Damon Fenacci <dfenacci at openjdk.org> wrote:

> # Issue
> An intermittent _Memory Pool not found_ error has been noticed when running  a few tests (_vmTestbase/vm/mlvm/meth/stress/compiler/deoptimize/Test.java_, _vmTestbase/vm/mlvm/meth/stress/compiler/sequences/Test.java_) on _macosx_aarch64_ (production build) with non-segmented code cache.
> 
> ## Origin
> The issue originates from the fact that aarch64 architecture is a weakly ordered memory architecture, i.e. it _permits the observation and completion of memory accesses in a different order from the program order_.
> 
> More precisely: while calling `CodeHeapPool::get_memory_usage`, the `used` and `committed` variables are retrieved
> https://github.com/openjdk/jdk/blob/138542de7889e8002df0e15a79e31d824c6a0473/src/hotspot/share/services/memoryPool.cpp#L181-L182
> and these are computed based on different variables saved in memory in `CodeCache::allocate` (during `heap->allocate` and `heap->expand_by` to be precise) .https://github.com/openjdk/jdk/blob/138542de7889e8002df0e15a79e31d824c6a0473/src/hotspot/share/code/codeCache.cpp#L535-L537
> The problem happens when first `heap->expand_by` gets called (which _increases_ `committed`) and then `heap->allocate` gets called in a second loop pass (which _increases_ `used`). Although stores in `CodeCache::allocate` happen in the this order, when reading from memory in `CodeHeapPool::get_memory_usage` it can happen that `used` has the newly computed value, while `committed` is still "old" (because of ARM’s weak memory order). This is a problem, since `committed` must be > than `used`.
> 
> # Solution
> 
> To avoid this situation we must assure that values used to calculate `committed` are actually saved before the values used to calculate `used` and that the opposite be true for reading. To enforce this we add an `acquire/release` ordering for `CodeHeap::_next_segment` ensuring that when `CodeHeap::allocated_capacity` is called, if `_next_segment` (which is used to calculate `used`) has already been updated in `CodeHeap::allocate`, then all values written before are going to be visible (the ones to calculate `committed`).
> 
> Acquiring a `CodeCache_lock` has been attempted in #15819 but resulted in deadlocks (that seem to be unavoidable).

src/hotspot/share/memory/heap.cpp line 556:

> 554: size_t CodeHeap::allocated_capacity() const {
> 555:   // size of used heap - size on freelist
> 556:   return segments_to_size(Atomic::load_acquire(&_next_segment) - _freelist_segments);

Suggestion:

  size_t next_segment = Atomic::load_acquire(&_next_segment);
  return segments_to_size(next_segment - _freelist_segments);

Reason: C++ can evaluate subexpressions in any order.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16143#discussion_r1354613759