Proper reader-writer synchronization in SensorCache

collectagent/collectagent.cpp

@@ -151,19 +151,15 @@ bool sensorGroupQueryCallback(const std::vector<string>& names, const uint64_t s
         DCDB::SensorId sid;
         // Creating a SID to perform the query
         if (sid.mqttTopicConvert(topic)) {
-            try {
-                mySensorCache.wait();
-                if (sensorMap.count(sid) > 0 && sensorMap[sid].getView(startTs, endTs, buffer, rel)) {
-                    // Data was found, can continue to next SID
-                    successCtr++;
-                } else {
-                    // This happens only if no data was found in the local cache
-                    topics.push_back(sid);
-                }
-            // To handle nasty (yet rare) race conditions on the sensor cache
-            } catch(const std::exception& e) {
+            mySensorCache.wait();
+            if (sensorMap.count(sid) > 0 && sensorMap[sid].getView(startTs, endTs, buffer, rel)) {
+                // Data was found, can continue to next SID
+                successCtr++;
+            } else {
+                // This happens only if no data was found in the local cache
                 topics.push_back(sid);
             }
+            mySensorCache.release();
         }
     }
     // If we are here then some sensors were not found in the cache - we need to fetch data from Cassandra
@@ -226,15 +222,13 @@ bool metadataQueryCallback(const string& name, SensorMetadata& buffer) {
     } catch(const std::domain_error& e) {}
     
     bool local = false;
-    try {
-        metadataStore->wait();
-        if(metadataStore->getMap().count(topic)) {
-            buffer = metadataStore->get(topic);
-            local = true;
-        }
+    metadataStore->wait();
+    if(metadataStore->getMap().count(topic)) {
+        buffer = metadataStore->get(topic);
+        local = true;
     }
-    catch(const std::exception& e) {}
-    
+    metadataStore->release();
+        
     if(!local) {
         // If we are here then the sensor was not found in the cache - we need to fetch data from Cassandra
         try {

collectagent/sensorcache.cpp

@@ -30,6 +30,7 @@
 SensorCache::SensorCache(uint64_t maxHistory) {
     this->_maxHistory = maxHistory;
     this->_updating.store(false);
+    this->_access.store(0);
 }
 
 SensorCache::~SensorCache() {
@@ -42,21 +43,22 @@ sensorCache_t& SensorCache::getSensorMap() {
 
 void SensorCache::storeSensor(SensorId sid, uint64_t ts, int64_t val) {
   reading_t s = { val, ts };
-  bool ownLock = false;
   /* Remove the reserved bytes to leverage the standard find function */
   sid.setRsvd(0);
-  try {
-      if (sensorCache.find(sid) == sensorCache.end()) {
-          // Spinning on the lock
-          while (_updating.exchange(true)) {}
-          ownLock = true;
-          sensorCache[sid] = CacheEntry(_maxHistory);
-          _updating.store(false);
-      }
+  wait();
+  auto sIt = sensorCache.find(sid);
+  if(sIt!=sensorCache.end()) {
+      CacheEntry& ce = sIt->second;
+      release();
+      ce.store(s);
+  } else {
+      release();
+      // Spinning on the lock
+      while (_updating.exchange(true)) {}
+      while(_access.load()>0) {}
+      sensorCache[sid] = CacheEntry(_maxHistory);
       sensorCache[sid].store(s);
-  } catch(const std::exception& e) {
-      if(ownLock)
-          _updating.store(false);
+      _updating.store(false);
   }
 }
 
@@ -153,6 +155,7 @@ uint64_t SensorCache::clean(uint64_t t) {
     uint64_t ctr = 0;
     // Spinning on the lock
     while (_updating.exchange(true)) {}
+    while(_access.load()>0) {}
     for (auto it = sensorCache.cbegin(); it != sensorCache.cend();) {
         uint64_t latestTs = it->second.getLatest().timestamp;
         if (latestTs!=0 && latestTs < thresh) {

collectagent/sensorcache.h

@@ -110,7 +110,14 @@ public:
 	*/
 	const void wait() {
 		while(_updating.load()) {}
-		return;
+		++_access;
+	}
+
+	/**
+	* @brief               Reduces the internal reading counter.
+	*/
+	const void release() {
+		--_access;
 	}
 
     /**
@@ -134,6 +141,7 @@ private:
 	uint64_t _maxHistory;
 	// Spinlock to regulate map modifications
 	std::atomic<bool> _updating;
+	std::atomic<int> _access;
 
 };
 

common/include/metadatastore.h

@@ -324,6 +324,7 @@ public:
      */
     MetadataStore() {
         _updating.store(false);
+        _access.store(0);
     }
     
     /**
@@ -352,7 +353,14 @@ public:
      */
     const void wait() {
         while(_updating.load()) {}
-        return;
+        ++_access;
+    }
+
+    /**
+     * @brief               Reduces the internal reading counter.
+     */
+    const void release() {
+        --_access;
     }
     
     /**
@@ -367,6 +375,7 @@ public:
     bool store(const string& key, const SensorMetadata& s) {
         // Spinlock to update the metadata store
         while(_updating.exchange(true)) {}
+        while(_access.load()>0) {}
         bool overwritten = !_metadata.count(key);
         _metadata[key] = s;
         _updating.store(false);
@@ -414,9 +423,16 @@ public:
      * @return              A reference to a sensorMetadata_t object
      */
     const SensorMetadata& get(const string& key) {
-        if(!_metadata.count(key))
+        wait();
+        auto it = _metadata.find(key);
+        if(it==_metadata.end()) {
+            release();
             throw invalid_argument("MetadataStore: key " + key + " does not exist!");
-        return _metadata[key];
+        } else {
+            const SensorMetadata& sm = it->second;
+            release();
+            return sm;
+        }
     }
 
     /**
@@ -431,13 +447,10 @@ public:
      */
     int64_t getTTL(const string& key) {
         int64_t ttl = 0;
-        try {
-            wait();
-            auto it = _metadata.find(key);
-            ttl = it==_metadata.end() || !it->second.getTTL() ? -1 : *it->second.getTTL()/1000000000;
-        } catch(const std::exception& e) {
-            ttl = -1;
-        }
+        wait();
+        auto it = _metadata.find(key);
+        ttl = it==_metadata.end() || !it->second.getTTL() ? -1 : *it->second.getTTL()/1000000000;
+        release();
         return ttl;
     }
     
@@ -469,6 +482,7 @@ protected:
     
     unordered_map<string, SensorMetadata> _metadata;
     atomic<bool> _updating;
+    atomic<int>  _access;
     
 };