Addressed PR comments and fixed a bug.
We now hold the mutex for both map insertions, to protect
against a concurrent GC that removes from the seconary map
before we can insert into the weak map.
diff --git a/ruby/ext/google/protobuf_c/protobuf.c b/ruby/ext/google/protobuf_c/protobuf.c
index 850f5e9..65263a4 100644
--- a/ruby/ext/google/protobuf_c/protobuf.c
+++ b/ruby/ext/google/protobuf_c/protobuf.c
@@ -259,17 +259,17 @@
// Lambda that will GC entries from the secondary map that are no longer present
// in the primary map.
-VALUE gc_secondary_map = Qnil;
+VALUE gc_secondary_map_lambda = Qnil;
ID length;
extern VALUE weak_obj_cache;
static void SecondaryMap_Init() {
rb_gc_register_address(&secondary_map);
- rb_gc_register_address(&gc_secondary_map);
+ rb_gc_register_address(&gc_secondary_map_lambda);
rb_gc_register_address(&secondary_map_mutex);
secondary_map = rb_hash_new();
- gc_secondary_map = rb_eval_string(
+ gc_secondary_map_lambda = rb_eval_string(
"->(secondary, weak) {\n"
" secondary.delete_if { |k, v| !weak.key?(v) }\n"
"}\n");
@@ -287,43 +287,61 @@
// secondary map that are no longer present in the WeakMap. The logic of
// how often to perform this GC is an artbirary tuning parameter that
// represents a straightforward CPU/memory tradeoff.
+//
+// Requires: secondary_map_mutex is held.
static void SecondaryMap_MaybeGC() {
+ PBRUBY_ASSERT(rb_mutex_locked_p(secondary_map_mutex) == Qtrue);
size_t weak_len = NUM2ULL(rb_funcall(weak_obj_cache, length, 0));
size_t secondary_len = RHASH_SIZE(secondary_map);
+ if (secondary_len < weak_len) {
+ // Logically this case should not be possible: a valid entry cannot exist in
+ // the weak table unless there is a corresponding entry in the secondary
+ // table. It should *always* be the case that secondary_len >= weak_len.
+ //
+ // However ObjectSpace::WeakMap#length (and therefore weak_len) is
+ // unreliable: it overreports its true length by including non-live objects.
+ // However these non-live objects are not yielded in iteration, so we may
+ // have previously deleted them from the secondary map in a previous
+ // invocation of SecondaryMap_MaybeGC().
+ //
+ // In this case, we can't measure any waste, so we just return.
+ return;
+ }
size_t waste = secondary_len - weak_len;
- PBRUBY_ASSERT(secondary_len >= weak_len);
// GC if we could remove at least 2000 entries or 20% of the table size
// (whichever is greater). Since the cost of the GC pass is O(N), we
// want to make sure that we condition this on overall table size, to
// avoid O(N^2) CPU costs.
size_t threshold = PBRUBY_MAX(secondary_len * 0.2, 2000);
if (waste > threshold) {
- rb_funcall(gc_secondary_map, rb_intern("call"), 2, secondary_map, weak_obj_cache);
+ rb_funcall(gc_secondary_map_lambda, rb_intern("call"), 2,
+ secondary_map, weak_obj_cache);
}
}
-static VALUE SecondaryMap_Get(VALUE key) {
+// Requires: secondary_map_mutex is held by this thread iff create == true.
+static VALUE SecondaryMap_Get(VALUE key, bool create) {
+ PBRUBY_ASSERT(!create || rb_mutex_locked_p(secondary_map_mutex) == Qtrue);
VALUE ret = rb_hash_lookup(secondary_map, key);
- if (ret == Qnil) {
- rb_mutex_lock(secondary_map_mutex);
+ if (ret == Qnil && create) {
SecondaryMap_MaybeGC();
ret = rb_eval_string("Object.new");
rb_hash_aset(secondary_map, key, ret);
- rb_mutex_unlock(secondary_map_mutex);
}
return ret;
}
#endif
-static VALUE ObjectCache_GetKey(const void* key) {
+// Requires: secondary_map_mutex is held by this thread iff create == true.
+static VALUE ObjectCache_GetKey(const void* key, bool create) {
char buf[sizeof(key)];
memcpy(&buf, &key, sizeof(key));
intptr_t key_int = (intptr_t)key;
PBRUBY_ASSERT((key_int & 3) == 0);
VALUE ret = LL2NUM(key_int >> 2);
#if USE_SECONDARY_MAP
- ret = SecondaryMap_Get(ret);
+ ret = SecondaryMap_Get(ret, create);
#endif
return ret;
}
@@ -347,14 +365,20 @@
void ObjectCache_Add(const void* key, VALUE val) {
PBRUBY_ASSERT(ObjectCache_Get(key) == Qnil);
- VALUE key_rb = ObjectCache_GetKey(key);
+#if USE_SECONDARY_MAP
+ rb_mutex_lock(secondary_map_mutex);
+#endif
+ VALUE key_rb = ObjectCache_GetKey(key, true);
rb_funcall(weak_obj_cache, item_set, 2, key_rb, val);
+#if USE_SECONDARY_MAP
+ rb_mutex_unlock(secondary_map_mutex);
+#endif
PBRUBY_ASSERT(ObjectCache_Get(key) == val);
}
// Returns the cached object for this key, if any. Otherwise returns Qnil.
VALUE ObjectCache_Get(const void* key) {
- VALUE key_rb = ObjectCache_GetKey(key);
+ VALUE key_rb = ObjectCache_GetKey(key, false);
return rb_funcall(weak_obj_cache, item_get, 1, key_rb);
}
