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

[Damon Fenacci]

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 explicit barriers are introduced: `OrderAccess::release` in `CodeCache::allocate` if we need to expand the heap, to ensure heap expansion is visible before we allocate, and `OrderAccess::acquire` before calculating `committed` in `CodeHeapPool::get_memory_usage` to make sure that the cache expansion is seen.
> 
> Acquiring a `CodeCache_lock` has been attempted in #15819 but resulted in deadlocks (that seem to be unavoidable).

This pull request has now been integrated.

Changeset: ddd07161
Author:    Damon Fenacci <dfenacci at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/ddd071617e7bc25c496973b231d02ced438d1344
Stats:     3 lines in 2 files changed: 3 ins; 0 del; 0 mod

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

Reviewed-by: dholmes, aph

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

PR: https://git.openjdk.org/jdk/pull/16143