Concurrency control on multi-cache additions.

src/6model/reprs/MVMMultiCache.c

@@ -130,23 +130,32 @@ MVMObject * MVM_multi_cache_add(MVMThreadContext *tc, MVMObject *cache_obj, MVMO
         MVM_exception_throw_adhoc(tc, "Multi cache addition requires an MVMCallCapture");
     }
 
+    /* If we're in a multi-threaded context, obtain the cache additions
+     * lock, and then do another lookup to ensure nobody beat us to
+     * making this entry. */
+    if (MVM_instance_have_user_threads(tc)) {
+        uv_mutex_lock(&(tc->instance->mutex_multi_cache_add));
+        if (MVM_multi_cache_find(tc, cache_obj, capture))
+            goto DONE;
+    }
+
     /* If it's zero arity, just stick it in that slot. */
     if (num_args == 0) {
         /* Can only be added if there are no named args */
         if (!has_nameds)
             MVM_ASSIGN_REF(tc, &(cache_obj->header), cache->zero_arity, result);
-        return cache_obj;
+        goto DONE;
     }
 
     /* If there's more args than the maximum, we can't cache it. */
     if (num_args > MVM_MULTICACHE_MAX_ARITY)
-        return cache_obj;
+        goto DONE;
 
     /* If the cache is saturated, don't do anything (we could instead do a random
      * replacement). */
     entries = cache->arity_caches[num_args - 1].num_entries;
     if (entries == MVM_MULTICACHE_MAX_ENTRIES)
-        return cache_obj;
+        goto DONE;
 
     /* Create arg tuple. */
     for (i = 0; i < num_args; i++) {
@@ -164,13 +173,13 @@ MVMObject * MVM_multi_cache_add(MVMThreadContext *tc, MVMObject *cache_obj, MVMO
                         }
                     }
                     else {
-                        return cache_obj;
+                        goto DONE;
                     }
                 }
                 arg_tup[i] = STABLE(arg)->type_cache_id | (IS_CONCRETE(arg) ? 1 : 0);
             }
             else {
-                return cache_obj;
+                goto DONE;
             }
         }
         else {
@@ -193,6 +202,11 @@ MVMObject * MVM_multi_cache_add(MVMThreadContext *tc, MVMObject *cache_obj, MVMO
     cache->arity_caches[num_args - 1].named_ok[entries] = has_nameds;
     cache->arity_caches[num_args - 1].num_entries = entries + 1;
 
+    /* Release lock if needed. */
+  DONE:
+    if (MVM_instance_have_user_threads(tc))
+        uv_mutex_unlock(&(tc->instance->mutex_multi_cache_add));
+
     /* Hand back the created/updated cache. */
     return cache_obj;
 }

src/core/instance.h

@@ -160,8 +160,10 @@ struct MVMInstance {
     /* int -> str cache */
     MVMString **int_to_str_cache;
 
-    /* Specialization installation mutex (global, as it's low contention, so
-     * no real motivation to have it more fine-grained at present). */
+    /* Multi-dispatch cache and specialization installation mutexes
+     * (global, as the additions are quite low contention, so no
+     * real motivation to have it more fine-grained at present). */
+    uv_mutex_t mutex_multi_cache_add;
     uv_mutex_t mutex_spesh_install;
 
     /* Log file for specializations, if we're to log them. */

src/moar.c

@@ -119,6 +119,9 @@ MVMInstance * MVM_vm_create_instance(void) {
      * them, so that spesh may end up optimizing more "internal" stuff. */
     MVM_callsite_initialize_common(instance->main_thread);
 
+    /* Multi-cache additions mutex. */
+    init_mutex(instance->mutex_multi_cache_add, "multi-cache addition");
+
     /* Mutex for spesh installations, and check if we've a file we
      * should log specializations to. */
     init_mutex(instance->mutex_spesh_install, "spesh installations");
@@ -136,6 +139,7 @@ MVMInstance * MVM_vm_create_instance(void) {
             instance->spesh_osr_enabled = 1;
     }
 
+    /* JIT environment/logging setup. */
     jit_disable = getenv("MVM_JIT_DISABLE");
     if (!jit_disable || strlen(jit_disable) == 0)
         instance->jit_enabled = 1;
@@ -298,6 +302,9 @@ void MVM_vm_destroy_instance(MVMInstance *instance) {
     /* Clean up Hash of hashes of symbol tables per hll. */
     uv_mutex_destroy(&instance->mutex_hll_syms);
 
+    /* Clean up multi cache addition mutex. */
+    uv_mutex_destroy(&instance->mutex_multi_cache_add);
+
     /* Clean up spesh install mutex and close any log. */
     uv_mutex_destroy(&instance->mutex_spesh_install);
     if (instance->spesh_log_fh)