Use Julia's finalizer implementation (#22)

This PR moves code about registering and running finalizers to `gc-common`.

Author: qinsoon

src/gc-common.c

@@ -49,6 +49,14 @@ memsize_t max_total_memory = (memsize_t) 2 * 1024 * 1024 * 1024;
 // finalizers
 // ---
 uint64_t finalizer_rngState[JL_RNG_SIZE];
+jl_mutex_t finalizers_lock;
+// `ptls->finalizers` and `finalizer_list_marked` might have tagged pointers.
+// If an object pointer has the lowest bit set, the next pointer is an unboxed c function pointer.
+// If an object pointer has the second lowest bit set, the current pointer is a c object pointer.
+//   It must be aligned at least 4, and it finalized immediately (at "quiescence").
+// `to_finalize` should not have tagged pointers.
+arraylist_t finalizer_list_marked;
+arraylist_t to_finalize;
 
 void jl_rng_split(uint64_t dst[JL_RNG_SIZE], uint64_t src[JL_RNG_SIZE]) JL_NOTSAFEPOINT;
 
@@ -57,6 +65,25 @@ JL_DLLEXPORT void jl_gc_init_finalizer_rng_state(void)
     jl_rng_split(finalizer_rngState, jl_current_task->rngState);
 }
 
+// The first two entries are assumed to be empty and the rest are assumed to
+// be pointers to `jl_value_t` objects
+STATIC_INLINE void jl_gc_push_arraylist(jl_task_t *ct, arraylist_t *list) JL_NOTSAFEPOINT
+{
+    void **items = list->items;
+    items[0] = (void*)JL_GC_ENCODE_PUSHARGS(list->len - 2);
+    items[1] = ct->gcstack;
+    ct->gcstack = (jl_gcframe_t*)items;
+}
+
+STATIC_INLINE void schedule_finalization(void *o, void *f) JL_NOTSAFEPOINT
+{
+    arraylist_push(&to_finalize, o);
+    arraylist_push(&to_finalize, f);
+    // doesn't need release, since we'll keep checking (on the reader) until we see the work and
+    // release our lock, and that will have a release barrier by then
+    jl_atomic_store_relaxed(&jl_gc_have_pending_finalizers, 1);
+}
+
 void run_finalizer(jl_task_t *ct, void *o, void *ff)
 {
     int ptr_finalizer = gc_ptr_tag(o, 1);
@@ -79,6 +106,243 @@ void run_finalizer(jl_task_t *ct, void *o, void *ff)
     }
 }
 
+void jl_gc_add_finalizer_(jl_ptls_t ptls, void *v, void *f) JL_NOTSAFEPOINT
+{
+    assert(jl_atomic_load_relaxed(&ptls->gc_state) == 0);
+    arraylist_t *a = &ptls->finalizers;
+    // This acquire load and the release store at the end are used to
+    // synchronize with `finalize_object` on another thread. Apart from the GC,
+    // which is blocked by entering a unsafe region, there might be only
+    // one other thread accessing our list in `finalize_object`
+    // (only one thread since it needs to acquire the finalizer lock).
+    // Similar to `finalize_object`, all content mutation has to be done
+    // between the acquire and the release of the length.
+    size_t oldlen = jl_atomic_load_acquire((_Atomic(size_t)*)&a->len);
+    if (__unlikely(oldlen + 2 > a->max)) {
+        JL_LOCK_NOGC(&finalizers_lock);
+        // `a->len` might have been modified.
+        // Another possibility is to always grow the array to `oldlen + 2` but
+        // it's simpler this way and uses slightly less memory =)
+        oldlen = a->len;
+        arraylist_grow(a, 2);
+        a->len = oldlen;
+        JL_UNLOCK_NOGC(&finalizers_lock);
+    }
+    void **items = a->items;
+    items[oldlen] = v;
+    items[oldlen + 1] = f;
+    jl_atomic_store_release((_Atomic(size_t)*)&a->len, oldlen + 2);
+}
+
+// Same assumption as `jl_gc_push_arraylist`. Requires the finalizers lock
+// to be hold for the current thread and will release the lock when the
+// function returns.
+void jl_gc_run_finalizers_in_list(jl_task_t *ct, arraylist_t *list) JL_NOTSAFEPOINT_LEAVE
+{
+    // Avoid marking `ct` as non-migratable via an `@async` task (as noted in the docstring
+    // of `finalizer`) in a finalizer:
+    uint8_t sticky = ct->sticky;
+    // empty out the first two entries for the GC frame
+    arraylist_push(list, list->items[0]);
+    arraylist_push(list, list->items[1]);
+    jl_gc_push_arraylist(ct, list);
+    void **items = list->items;
+    size_t len = list->len;
+    JL_UNLOCK_NOGC(&finalizers_lock);
+    // run finalizers in reverse order they were added, so lower-level finalizers run last
+    for (size_t i = len-4; i >= 2; i -= 2)
+        run_finalizer(ct, items[i], items[i + 1]);
+    // first entries were moved last to make room for GC frame metadata
+    run_finalizer(ct, items[len-2], items[len-1]);
+    // matches the jl_gc_push_arraylist above
+    JL_GC_POP();
+    ct->sticky = sticky;
+}
+
+void run_finalizers(jl_task_t *ct)
+{
+    // Racy fast path:
+    // The race here should be OK since the race can only happen if
+    // another thread is writing to it with the lock held. In such case,
+    // we don't need to run pending finalizers since the writer thread
+    // will flush it.
+    if (to_finalize.len == 0)
+        return;
+    JL_LOCK_NOGC(&finalizers_lock);
+    if (to_finalize.len == 0) {
+        JL_UNLOCK_NOGC(&finalizers_lock);
+        return;
+    }
+    arraylist_t copied_list;
+    memcpy(&copied_list, &to_finalize, sizeof(copied_list));
+    if (to_finalize.items == to_finalize._space) {
+        copied_list.items = copied_list._space;
+    }
+    jl_atomic_store_relaxed(&jl_gc_have_pending_finalizers, 0);
+    arraylist_new(&to_finalize, 0);
+
+    uint64_t save_rngState[JL_RNG_SIZE];
+    memcpy(&save_rngState[0], &ct->rngState[0], sizeof(save_rngState));
+    jl_rng_split(ct->rngState, finalizer_rngState);
+
+    // This releases the finalizers lock.
+    int8_t was_in_finalizer = ct->ptls->in_finalizer;
+    ct->ptls->in_finalizer = 1;
+    jl_gc_run_finalizers_in_list(ct, &copied_list);
+    ct->ptls->in_finalizer = was_in_finalizer;
+    arraylist_free(&copied_list);
+
+    memcpy(&ct->rngState[0], &save_rngState[0], sizeof(save_rngState));
+}
+
+// if `need_sync` is true, the `list` is the `finalizers` list of another
+// thread and we need additional synchronizations
+void finalize_object(arraylist_t *list, jl_value_t *o,
+                            arraylist_t *copied_list, int need_sync) JL_NOTSAFEPOINT
+{
+    // The acquire load makes sure that the first `len` objects are valid.
+    // If `need_sync` is true, all mutations of the content should be limited
+    // to the first `oldlen` elements and no mutation is allowed after the
+    // new length is published with the `cmpxchg` at the end of the function.
+    // This way, the mutation should not conflict with the owning thread,
+    // which only writes to locations later than `len`
+    // and will not resize the buffer without acquiring the lock.
+    size_t len = need_sync ? jl_atomic_load_acquire((_Atomic(size_t)*)&list->len) : list->len;
+    size_t oldlen = len;
+    void **items = list->items;
+    size_t j = 0;
+    for (size_t i = 0; i < len; i += 2) {
+        void *v = items[i];
+        int move = 0;
+        if (o == (jl_value_t*)gc_ptr_clear_tag(v, 1)) {
+            void *f = items[i + 1];
+            move = 1;
+            arraylist_push(copied_list, v);
+            arraylist_push(copied_list, f);
+        }
+        if (move || __unlikely(!v)) {
+            // remove item
+        }
+        else {
+            if (j < i) {
+                items[j] = items[i];
+                items[j+1] = items[i+1];
+            }
+            j += 2;
+        }
+    }
+    len = j;
+    if (oldlen == len)
+        return;
+    if (need_sync) {
+        // The memset needs to be unconditional since the thread might have
+        // already read the length.
+        // The `memset` (like any other content mutation) has to be done
+        // **before** the `cmpxchg` which publishes the length.
+        memset(&items[len], 0, (oldlen - len) * sizeof(void*));
+        jl_atomic_cmpswap((_Atomic(size_t)*)&list->len, &oldlen, len);
+    }
+    else {
+        list->len = len;
+    }
+}
+
+JL_DLLEXPORT void jl_gc_add_ptr_finalizer(jl_ptls_t ptls, jl_value_t *v, void *f) JL_NOTSAFEPOINT
+{
+    jl_gc_add_finalizer_(ptls, (void*)(((uintptr_t)v) | 1), f);
+}
+
+// schedule f(v) to call at the next quiescent interval (aka after the next safepoint/region on all threads)
+JL_DLLEXPORT void jl_gc_add_quiescent(jl_ptls_t ptls, void **v, void *f) JL_NOTSAFEPOINT
+{
+    assert(!gc_ptr_tag(v, 3));
+    jl_gc_add_finalizer_(ptls, (void*)(((uintptr_t)v) | 3), f);
+}
+
+JL_DLLEXPORT void jl_gc_add_finalizer_th(jl_ptls_t ptls, jl_value_t *v, jl_function_t *f) JL_NOTSAFEPOINT
+{
+    if (__unlikely(jl_typetagis(f, jl_voidpointer_type))) {
+        jl_gc_add_ptr_finalizer(ptls, v, jl_unbox_voidpointer(f));
+    }
+    else {
+        jl_gc_add_finalizer_(ptls, v, f);
+    }
+}
+
+JL_DLLEXPORT void jl_gc_run_pending_finalizers(jl_task_t *ct)
+{
+    if (ct == NULL)
+        ct = jl_current_task;
+    jl_ptls_t ptls = ct->ptls;
+    if (!ptls->in_finalizer && ptls->locks.len == 0 && ptls->finalizers_inhibited == 0) {
+        run_finalizers(ct);
+    }
+}
+
+JL_DLLEXPORT void jl_finalize_th(jl_task_t *ct, jl_value_t *o)
+{
+    JL_LOCK_NOGC(&finalizers_lock);
+    // Copy the finalizers into a temporary list so that code in the finalizer
+    // won't change the list as we loop through them.
+    // This list is also used as the GC frame when we are running the finalizers
+    arraylist_t copied_list;
+    arraylist_new(&copied_list, 0);
+    // No need to check the to_finalize list since the user is apparently
+    // still holding a reference to the object
+    int gc_n_threads;
+    jl_ptls_t* gc_all_tls_states;
+    gc_n_threads = jl_atomic_load_acquire(&jl_n_threads);
+    gc_all_tls_states = jl_atomic_load_relaxed(&jl_all_tls_states);
+    for (int i = 0; i < gc_n_threads; i++) {
+        jl_ptls_t ptls2 = gc_all_tls_states[i];
+        if (ptls2 != NULL)
+            finalize_object(&ptls2->finalizers, o, &copied_list, jl_atomic_load_relaxed(&ct->tid) != i);
+    }
+    finalize_object(&finalizer_list_marked, o, &copied_list, 0);
+    gc_n_threads = 0;
+    gc_all_tls_states = NULL;
+    if (copied_list.len > 0) {
+        // This releases the finalizers lock.
+        jl_gc_run_finalizers_in_list(ct, &copied_list);
+    }
+    else {
+        JL_UNLOCK_NOGC(&finalizers_lock);
+    }
+    arraylist_free(&copied_list);
+}
+
+void schedule_all_finalizers(arraylist_t *flist) JL_NOTSAFEPOINT
+{
+    void **items = flist->items;
+    size_t len = flist->len;
+    for(size_t i = 0; i < len; i+=2) {
+        void *v = items[i];
+        void *f = items[i + 1];
+        if (__unlikely(!v))
+            continue;
+        schedule_finalization(v, f);
+    }
+    flist->len = 0;
+}
+
+void jl_gc_run_all_finalizers(jl_task_t *ct)
+{
+    if (!ct) return;
+    int gc_n_threads;
+    jl_ptls_t* gc_all_tls_states;
+    gc_n_threads = jl_atomic_load_acquire(&jl_n_threads);
+    gc_all_tls_states = jl_atomic_load_relaxed(&jl_all_tls_states);
+    schedule_all_finalizers(&finalizer_list_marked);
+    for (int i = 0; i < gc_n_threads; i++) {
+        jl_ptls_t ptls2 = gc_all_tls_states[i];
+        if (ptls2 != NULL)
+            schedule_all_finalizers(&ptls2->finalizers);
+    }
+    gc_n_threads = 0;
+    gc_all_tls_states = NULL;
+    run_finalizers(ct);
+}
+
 JL_DLLEXPORT int jl_gc_get_finalizers_inhibited(jl_ptls_t ptls)
 {
     if (ptls == NULL)