Merge pull request #32 from LightBitsLabs/anton/create_lookup_unlocked

lookup without holding the lock during registration

Author: ron-lb

src/procstat.c

@@ -54,6 +54,14 @@
 #define ARRAY_SIZE(a) (sizeof(a) / sizeof(*a))
 #endif
 
+#ifndef unlikely
+#define unlikely(x)     __builtin_expect(!!(x), 0)
+#endif
+
+#ifndef likely
+#define likely(x)       __builtin_expect(!!(x), 1)
+#endif
+
 enum {
 	STATS_ENTRY_FLAG_REGISTERED  = 1 << 0,
 	STATS_ENTRY_FLAG_DIR	     = 1 << 1,
@@ -771,17 +779,32 @@ static int register_item(struct procstat_context *context,
 			 struct procstat_item *item,
 			 struct procstat_directory *parent)
 {
-	pthread_mutex_lock(&context->global_lock);
-	if (parent) {
+	if (likely(parent)) {
 		struct procstat_item *duplicate;
+		bool locked = false;
+
+		if (unlikely(root_directory(context, parent))) {
+			/* Only root directory can be modified concurrently by different threads */
+			pthread_mutex_lock(&context->global_lock);
+			locked = true;
+		}
 
 		duplicate = lookup_item_locked(parent, procstat_item_name(item), item->name_hash);
-		if (duplicate) {
-			pthread_mutex_unlock(&context->global_lock);
+		if (unlikely(duplicate)) {
+			if (locked)
+				pthread_mutex_unlock(&context->global_lock);
 			return EEXIST;
 		}
+
+		if (likely(!locked))
+			pthread_mutex_lock(&context->global_lock);
+
 		list_add_tail(&item->entry, &parent->children);
+	} else {
+		pthread_mutex_lock(&context->global_lock);
 	}
+
+
 	item->flags |= STATS_ENTRY_FLAG_REGISTERED;
 	item->refcnt = 1;
 	item->parent = parent;