hakmem/core/box/free_local_box.c

#include "free_local_box.h"
#include "free_publish_box.h"
#include "hakmem_tiny.h"

void tiny_free_local_box(SuperSlab* ss, int slab_idx, TinySlabMeta* meta, void* ptr, uint32_t my_tid) {
    extern _Atomic uint64_t g_free_local_box_calls;
    atomic_fetch_add_explicit(&g_free_local_box_calls, 1, memory_order_relaxed);
    if (!(ss && ss->magic == SUPERSLAB_MAGIC)) return;
    if (slab_idx < 0 || slab_idx >= ss_slabs_capacity(ss)) return;
    (void)my_tid;

    if (__builtin_expect(tiny_refill_failfast_level() >= 2, 0)) {
        int actual_idx = slab_index_for(ss, ptr);
        if (actual_idx != slab_idx) {
            tiny_failfast_abort_ptr("free_local_box_idx", ss, slab_idx, ptr, "slab_idx_mismatch");
        } else {
            size_t blk = g_tiny_class_sizes[ss->size_class];
            uint8_t* slab_base = tiny_slab_base_for(ss, slab_idx);
            uintptr_t delta = (uintptr_t)ptr - (uintptr_t)slab_base;
            if (blk == 0 || (delta % blk) != 0) {
                tiny_failfast_abort_ptr("free_local_box_align", ss, slab_idx, ptr, "misaligned");
            } else if (meta && delta / blk >= meta->capacity) {
                tiny_failfast_abort_ptr("free_local_box_range", ss, slab_idx, ptr, "out_of_capacity");
            }
        }
    }

    void* prev = meta->freelist;
    *(void**)ptr = prev;
    meta->freelist = ptr;
    tiny_failfast_log("free_local_box", ss->size_class, ss, meta, ptr, prev);
    // BUGFIX: Memory barrier to ensure freelist visibility before used decrement
    // Without this, other threads can see new freelist but old used count (race)
    atomic_thread_fence(memory_order_release);

    // Optional freelist mask update on first push
    do {
        static int g_mask_en = -1;
        if (__builtin_expect(g_mask_en == -1, 0)) {
            const char* e = getenv("HAKMEM_TINY_FREELIST_MASK");
            g_mask_en = (e && *e && *e != '0') ? 1 : 0;
        }
        if (__builtin_expect(g_mask_en, 0) && prev == NULL) {
            uint32_t bit = (1u << slab_idx);
            atomic_fetch_or_explicit(&ss->freelist_mask, bit, memory_order_release);
        }
    } while (0);

    // Track local free (debug helpers may be no-op)
    tiny_remote_track_on_local_free(ss, slab_idx, ptr, "local_free", my_tid);
    meta->used--;
    ss_active_dec_one(ss);

    if (prev == NULL) {
        // First-free → advertise slab to adopters
        tiny_free_publish_first_free((int)ss->size_class, ss, slab_idx);
    }
}