Overview

Copying a BSONObj increments the SharedBuffer holder's refcount. Atomic::fetchAndAdd() defaults to seq_cst, which on aarch64 emits a full acquire-release LDADDAL. Relaxed is sufficient for an increment because the caller already holds a reference, so only the decrement that reaches zero needs ordering. RefCountable already does exactly this, with the same justification.

Scope of Work

* src/mongo/util/shared_buffer.h — use fetchAndAddRelaxed() in intrusive_ptr_add_ref()

Notes

Found via getFieldDottedOrArray(), which takes an owning BSONObj copy on the matcher's per-document path. With PGO and BOLT enabled that function is inlined into BSONMatchableDocument::allocateIterator, the third hottest symbol at ~3%, and the atomic dominated it.

The decrement is deliberately left at seq_cst. Relaxing it to release with an acquire fence on the zero transition has no in-tree precedent, and on aarch64 acq_rel and seq_cst emit the same instruction, so it needs a separate argument.

Acceptance Criteria

* No change in behaviour; TSAN clean