Merge "Fix a race PingPongTransactionChain"
authorTony Tkacik <ttkacik@cisco.com>
Thu, 15 Jan 2015 10:12:10 +0000 (10:12 +0000)
committerGerrit Code Review <gerrit@opendaylight.org>
Thu, 15 Jan 2015 10:12:10 +0000 (10:12 +0000)
opendaylight/md-sal/sal-dom-broker/src/main/java/org/opendaylight/controller/md/sal/dom/broker/impl/PingPongTransactionChain.java

index abd348a..c3a56ed 100644 (file)
@@ -64,7 +64,6 @@ public final class PingPongTransactionChain implements DOMTransactionChain {
      */
     private static final AtomicReferenceFieldUpdater<PingPongTransactionChain, PingPongTransaction> READY_UPDATER =
             AtomicReferenceFieldUpdater.newUpdater(PingPongTransactionChain.class, PingPongTransaction.class, "readyTx");
-    @SuppressWarnings("unused") // Accessed via READY_UPDATER
     private volatile PingPongTransaction readyTx;
 
     /**
@@ -157,7 +156,10 @@ public final class PingPongTransactionChain implements DOMTransactionChain {
         return oldTx;
     }
 
-    // This forces allocateTransaction() on a slow path
+    /*
+     * This forces allocateTransaction() on a slow path, which has to happen after
+     * this method has completed executing.
+     */
     @GuardedBy("this")
     private void processIfReady() {
         final PingPongTransaction tx = READY_UPDATER.getAndSet(this, null);
@@ -222,14 +224,28 @@ public final class PingPongTransactionChain implements DOMTransactionChain {
     }
 
     private void readyTransaction(final @Nonnull PingPongTransaction tx) {
+        // First mark the transaction as not locked.
         final boolean lockedMatch = LOCKED_UPDATER.compareAndSet(this, tx, null);
         Preconditions.checkState(lockedMatch, "Attempted to submit transaction %s while we have %s", tx, lockedTx);
-
         LOG.debug("Transaction {} unlocked", tx);
 
+        /*
+         * The transaction is ready. It will then be picked up by either next allocation,
+         * or a background transaction completion callback.
+         */
+        final boolean success = READY_UPDATER.compareAndSet(this, null, tx);
+        Preconditions.checkState(success, "Transaction %s collided on ready state", tx, readyTx);
+        LOG.debug("Transaction {} readied");
+
+        /*
+         * We do not see a transaction being in-flight, so we need to take care of dispatching
+         * the transaction to the backend. We are in the ready case, we cannot short-cut
+         * the checking of readyTx, as an in-flight transaction may have completed between us
+         * setting the field above and us checking.
+         */
         if (inflightTx == null) {
             synchronized (this) {
-                processTransaction(tx);
+                processIfReady();
             }
         }
     }

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