Refactor vm_lookup_cc to allow lock-free lookups in RClass.cc_tbl

In multi-ractor mode, the `cc_tbl` mutations use the RCU pattern,
which allow lock-less reads.

Based on the assumption that invalidations and misses should be
increasingly rare as the process ages, locking on modification
isn't a big concern.

Author: byroot

bootstraptest/test_ractor.rb

@@ -2315,3 +2315,33 @@ def Warning.warn(msg)
   raise unless $msg.all?{/Ractor#take/ =~ it}
   $msg.size
 }
+
+# Cause lots of inline CC misses.
+assert_equal 'ok', <<~'RUBY'
+  class A; def test; 1 + 1; end; end
+  class B; def test; 1 + 1; end; end
+  class C; def test; 1 + 1; end; end
+  class D; def test; 1 + 1; end; end
+  class E; def test; 1 + 1; end; end
+  class F; def test; 1 + 1; end; end
+  class G; def test; 1 + 1; end; end
+
+  objs = [A.new, B.new, C.new, D.new, E.new, F.new, G.new].freeze
+
+  def call_test(obj)
+    obj.test
+  end
+
+  ractors = 7.times.map do
+    Ractor.new(objs) do |objs|
+      objs = objs.shuffle
+      100_000.times do
+        objs.each do |o|
+          call_test(o)
+        end
+      end
+    end
+  end
+  ractors.each(&:join)
+  :ok
+RUBY

id_table.c

@@ -395,7 +395,7 @@ VALUE
 rb_managed_id_table_dup(VALUE old_table)
 {
     struct rb_id_table *new_tbl;
-    VALUE obj = TypedData_Make_Struct(0, struct rb_id_table, &rb_managed_id_table_type, new_tbl);
+    VALUE obj = TypedData_Make_Struct(0, struct rb_id_table, RTYPEDDATA_TYPE(old_table), new_tbl);
     struct rb_id_table *old_tbl = managed_id_table_ptr(old_table);
     rb_id_table_init(new_tbl, old_tbl->num + 1);
     rb_id_table_foreach(old_tbl, managed_id_table_dup_i, new_tbl);

imemo.c

@@ -515,41 +515,6 @@ rb_free_const_table(struct rb_id_table *tbl)
     rb_id_table_free(tbl);
 }
 
-// alive: if false, target pointers can be freed already.
-static void
-vm_ccs_free(struct rb_class_cc_entries *ccs, int alive, VALUE klass)
-{
-    if (ccs->entries) {
-        for (int i=0; i<ccs->len; i++) {
-            const struct rb_callcache *cc = ccs->entries[i].cc;
-            if (!alive) {
-                // ccs can be free'ed.
-                if (rb_gc_pointer_to_heap_p((VALUE)cc) &&
-                    !rb_objspace_garbage_object_p((VALUE)cc) &&
-                    IMEMO_TYPE_P(cc, imemo_callcache) &&
-                    cc->klass == klass) {
-                    // OK. maybe target cc.
-                }
-                else {
-                    continue;
-                }
-            }
-
-            VM_ASSERT(!vm_cc_super_p(cc) && !vm_cc_refinement_p(cc));
-            vm_cc_invalidate(cc);
-        }
-        ruby_xfree(ccs->entries);
-    }
-    ruby_xfree(ccs);
-}
-
-void
-rb_vm_ccs_free(struct rb_class_cc_entries *ccs)
-{
-    RB_DEBUG_COUNTER_INC(ccs_free);
-    vm_ccs_free(ccs, true, Qundef);
-}
-
 static inline void
 imemo_fields_free(struct rb_fields *fields)
 {

vm_callinfo.h

@@ -330,6 +330,8 @@ cc_check_class(VALUE klass)
 }
 
 VALUE rb_vm_cc_table_create(size_t capa);
+VALUE rb_vm_cc_table_dup(VALUE old_table);
+void rb_vm_cc_table_delete(VALUE table, ID mid);
 
 static inline const struct rb_callcache *
 vm_cc_new(VALUE klass,
@@ -600,11 +602,14 @@ vm_ccs_p(const struct rb_class_cc_entries *ccs)
 static inline bool
 vm_cc_check_cme(const struct rb_callcache *cc, const rb_callable_method_entry_t *cme)
 {
-    if (vm_cc_cme(cc) == cme ||
-        (cme->def->iseq_overload && vm_cc_cme(cc) == rb_vm_lookup_overloaded_cme(cme))) {
+    bool valid;
+    RB_VM_LOCKING() {
+        valid = vm_cc_cme(cc) == cme ||
+            (cme->def->iseq_overload && vm_cc_cme(cc) == rb_vm_lookup_overloaded_cme(cme));
+    }
+    if (valid) {
         return true;
     }
-    else {
 #if 1
         // debug print
 
@@ -616,13 +621,9 @@ vm_cc_check_cme(const struct rb_callcache *cc, const rb_callable_method_entry_t
         rp(vm_cc_cme(cc));
         rp(rb_vm_lookup_overloaded_cme(cme));
 #endif
-        return false;
-    }
+    return false;
 }
 
 #endif
 
-// gc.c
-void rb_vm_ccs_free(struct rb_class_cc_entries *ccs);
-
 #endif /* RUBY_VM_CALLINFO_H */

vm_insnhelper.c

@@ -2060,58 +2060,49 @@ vm_ccs_verify(struct rb_class_cc_entries *ccs, ID mid, VALUE klass)
 
 const rb_callable_method_entry_t *rb_check_overloaded_cme(const rb_callable_method_entry_t *cme, const struct rb_callinfo * const ci);
 
-static const struct rb_callcache *
-vm_search_cc(const VALUE klass, const struct rb_callinfo * const ci)
+static void
+vm_evict_cc(VALUE klass, VALUE cc_tbl, ID mid)
 {
-    const ID mid = vm_ci_mid(ci);
-    VALUE cc_tbl = RCLASS_WRITABLE_CC_TBL(klass);
-    struct rb_class_cc_entries *ccs = NULL;
-    VALUE ccs_data;
+    ASSERT_vm_locking();
 
-    if (cc_tbl) {
-        // CCS data is keyed on method id, so we don't need the method id
-        // for doing comparisons in the `for` loop below.
-        if (rb_managed_id_table_lookup(cc_tbl, mid, &ccs_data)) {
-            ccs = (struct rb_class_cc_entries *)ccs_data;
-            const int ccs_len = ccs->len;
-
-            if (UNLIKELY(METHOD_ENTRY_INVALIDATED(ccs->cme))) {
-                rb_managed_id_table_delete(cc_tbl, mid);
-                rb_vm_ccs_free(ccs);
-                ccs = NULL;
-            }
-            else {
-                VM_ASSERT(vm_ccs_verify(ccs, mid, klass));
+    if (rb_multi_ractor_p()) {
+        if (RCLASS_WRITABLE_CC_TBL(klass) != cc_tbl) {
+            // Another ractor updated the CC table while we were waiting on the VM lock.
+            // We have to retry.
+            return;
+        }
 
-                // We already know the method id is correct because we had
-                // to look up the ccs_data by method id.  All we need to
-                // compare is argc and flag
-                unsigned int argc = vm_ci_argc(ci);
-                unsigned int flag = vm_ci_flag(ci);
+        struct rb_class_cc_entries *ccs = NULL;
+        rb_managed_id_table_lookup(cc_tbl, mid, (VALUE *)&ccs);
 
-                for (int i=0; i<ccs_len; i++) {
-                    unsigned int ccs_ci_argc = ccs->entries[i].argc;
-                    unsigned int ccs_ci_flag = ccs->entries[i].flag;
-                    const struct rb_callcache *ccs_cc = ccs->entries[i].cc;
+        if (!ccs || !METHOD_ENTRY_INVALIDATED(ccs->cme)) {
+            // Another ractor replaced that entry while we were waiting on the VM lock.
+            return;
+        }
 
-                    VM_ASSERT(IMEMO_TYPE_P(ccs_cc, imemo_callcache));
+        VALUE new_table = rb_vm_cc_table_dup(cc_tbl);
+        rb_vm_cc_table_delete(new_table, mid);
+        RB_OBJ_ATOMIC_WRITE(klass, &RCLASS_WRITABLE_CC_TBL(klass), new_table);
+    }
+    else {
+        rb_vm_cc_table_delete(cc_tbl, mid);
+    }
+}
 
-                    if (ccs_ci_argc == argc && ccs_ci_flag == flag) {
-                        RB_DEBUG_COUNTER_INC(cc_found_in_ccs);
+static const struct rb_callcache *
+vm_populate_cc(VALUE klass, const struct rb_callinfo * const ci, ID mid)
+{
+    ASSERT_vm_locking();
 
-                        VM_ASSERT(vm_cc_cme(ccs_cc)->called_id == mid);
-                        VM_ASSERT(ccs_cc->klass == klass);
-                        VM_ASSERT(!METHOD_ENTRY_INVALIDATED(vm_cc_cme(ccs_cc)));
+    VALUE cc_tbl = RCLASS_WRITABLE_CC_TBL(klass);
+    const VALUE original_cc_table = cc_tbl;
+    struct rb_class_cc_entries *ccs = NULL;
 
-                        return ccs_cc;
-                    }
-                }
-            }
-        }
+    if (!cc_tbl) {
+        cc_tbl = rb_vm_cc_table_create(1);
     }
-    else {
-        cc_tbl = rb_vm_cc_table_create(2);
-        RCLASS_WRITE_CC_TBL(klass, cc_tbl);
+    else if (rb_multi_ractor_p()) {
+        cc_tbl = rb_vm_cc_table_dup(cc_tbl);
     }
 
     RB_DEBUG_COUNTER_INC(cc_not_found_in_ccs);
@@ -2143,11 +2134,7 @@ vm_search_cc(const VALUE klass, const struct rb_callinfo * const ci)
     if (ccs == NULL) {
         VM_ASSERT(cc_tbl);
 
-        if (LIKELY(rb_managed_id_table_lookup(cc_tbl, mid, &ccs_data))) {
-            // rb_callable_method_entry() prepares ccs.
-            ccs = (struct rb_class_cc_entries *)ccs_data;
-        }
-        else {
+        if (!LIKELY(rb_managed_id_table_lookup(cc_tbl, mid, (VALUE *)&ccs))) {
             // TODO: required?
             ccs = vm_ccs_create(klass, cc_tbl, mid, cme);
         }
@@ -2162,6 +2149,91 @@ vm_search_cc(const VALUE klass, const struct rb_callinfo * const ci)
     VM_ASSERT(cme->called_id == mid);
     VM_ASSERT(vm_cc_cme(cc)->called_id == mid);
 
+    if (original_cc_table != cc_tbl) {
+        RB_OBJ_ATOMIC_WRITE(klass, &RCLASS_WRITABLE_CC_TBL(klass), cc_tbl);
+    }
+
+    return cc;
+}
+
+static const struct rb_callcache *
+vm_lookup_cc(const VALUE klass, const struct rb_callinfo * const ci, ID mid)
+{
+    VALUE cc_tbl;
+    struct rb_class_cc_entries *ccs;
+retry:
+    cc_tbl = RUBY_ATOMIC_VALUE_LOAD(RCLASS_WRITABLE_CC_TBL(klass));
+    ccs = NULL;
+
+    if (cc_tbl) {
+        // CCS data is keyed on method id, so we don't need the method id
+        // for doing comparisons in the `for` loop below.
+
+        if (rb_managed_id_table_lookup(cc_tbl, mid, (VALUE *)&ccs)) {
+            const int ccs_len = ccs->len;
+
+            if (UNLIKELY(METHOD_ENTRY_INVALIDATED(ccs->cme))) {
+                RB_VM_LOCKING() {
+                    vm_evict_cc(klass, cc_tbl, mid);
+                }
+                goto retry;
+            }
+            else {
+                VM_ASSERT(vm_ccs_verify(ccs, mid, klass));
+
+                // We already know the method id is correct because we had
+                // to look up the ccs_data by method id.  All we need to
+                // compare is argc and flag
+                unsigned int argc = vm_ci_argc(ci);
+                unsigned int flag = vm_ci_flag(ci);
+
+                for (int i=0; i<ccs_len; i++) {
+                    unsigned int ccs_ci_argc = ccs->entries[i].argc;
+                    unsigned int ccs_ci_flag = ccs->entries[i].flag;
+                    const struct rb_callcache *ccs_cc = ccs->entries[i].cc;
+
+                    VM_ASSERT(IMEMO_TYPE_P(ccs_cc, imemo_callcache));
+
+                    if (ccs_ci_argc == argc && ccs_ci_flag == flag) {
+                        RB_DEBUG_COUNTER_INC(cc_found_in_ccs);
+
+                        VM_ASSERT(vm_cc_cme(ccs_cc)->called_id == mid);
+                        VM_ASSERT(ccs_cc->klass == klass);
+                        VM_ASSERT(!METHOD_ENTRY_INVALIDATED(vm_cc_cme(ccs_cc)));
+
+                        return ccs_cc;
+                    }
+                }
+            }
+        }
+    }
+
+    RB_GC_GUARD(cc_tbl);
+    return NULL;
+}
+
+static const struct rb_callcache *
+vm_search_cc(const VALUE klass, const struct rb_callinfo * const ci)
+{
+    const ID mid = vm_ci_mid(ci);
+
+    const struct rb_callcache *cc = vm_lookup_cc(klass, ci, mid);
+    if (cc) {
+        return cc;
+    }
+
+    RB_VM_LOCKING() {
+        if (rb_multi_ractor_p()) {
+            // The CC may have been populated by another ractor while we were waiting on the lock,
+            // so we must lookup a second time.
+            cc = vm_lookup_cc(klass, ci, mid);
+        }
+
+        if (!cc) {
+            cc = vm_populate_cc(klass, ci, mid);
+        }
+    }
+
     return cc;
 }
 
@@ -2172,16 +2244,14 @@ rb_vm_search_method_slowpath(const struct rb_callinfo *ci, VALUE klass)
 
     VM_ASSERT_TYPE2(klass, T_CLASS, T_ICLASS);
 
-    RB_VM_LOCKING() {
-        cc = vm_search_cc(klass, ci);
+    cc = vm_search_cc(klass, ci);
 
-        VM_ASSERT(cc);
-        VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
-        VM_ASSERT(cc == vm_cc_empty() || cc->klass == klass);
-        VM_ASSERT(cc == vm_cc_empty() || callable_method_entry_p(vm_cc_cme(cc)));
-        VM_ASSERT(cc == vm_cc_empty() || !METHOD_ENTRY_INVALIDATED(vm_cc_cme(cc)));
-        VM_ASSERT(cc == vm_cc_empty() || vm_cc_cme(cc)->called_id == vm_ci_mid(ci));
-    }
+    VM_ASSERT(cc);
+    VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
+    VM_ASSERT(cc == vm_cc_empty() || cc->klass == klass);
+    VM_ASSERT(cc == vm_cc_empty() || callable_method_entry_p(vm_cc_cme(cc)));
+    VM_ASSERT(cc == vm_cc_empty() || !METHOD_ENTRY_INVALIDATED(vm_cc_cme(cc)));
+    VM_ASSERT(cc == vm_cc_empty() || vm_cc_cme(cc)->called_id == vm_ci_mid(ci));
 
     return cc;
 }