BFS: Make the log-flusher thread persistent.

Previously, a new log-flush thread was created every time there was
an idle transaction. Now we just release a semaphore for an
already-existing thread.

Change-Id: If788dbe17ef8e069ce12aa7b778626d051cce2d0

src/add-ons/kernel/file_systems/bfs/Journal.cpp

@@ -409,6 +409,12 @@ Journal::Journal(Volume* volume)
 {
 	recursive_lock_init(&fLock, "bfs journal");
 	mutex_init(&fEntriesLock, "bfs journal entries");
+
+	fLogFlusherSem = create_sem(0, "bfs log flusher");
+	fLogFlusher = spawn_kernel_thread(&Journal::_LogFlusher, "bfs log flusher",
+		B_NORMAL_PRIORITY, this);
+	if (fLogFlusher > 0)
+		resume_thread(fLogFlusher);
 }
 
 
@@ -418,6 +424,11 @@ Journal::~Journal()
 
 	recursive_lock_destroy(&fLock);
 	mutex_destroy(&fEntriesLock);
+
+	sem_id logFlusher = fLogFlusherSem;
+	fLogFlusherSem = -1;
+	delete_sem(logFlusher);
+	wait_for_thread(fLogFlusher, NULL);
 }
 
 
@@ -685,20 +696,24 @@ Journal::_TransactionWritten(int32 transactionID, int32 event, void* _logEntry)
 /*static*/ void
 Journal::_TransactionIdle(int32 transactionID, int32 event, void* _journal)
 {
-	// The current transaction seems to be idle - flush it. We can't do this
+	// The current transaction seems to be idle - flush it. (We can't do this
-	// in this thread, as flushing the log can produce new transaction events.
+	// in this thread, as flushing the log can produce new transaction events.)
-	thread_id id = spawn_kernel_thread(&Journal::_FlushLog, "bfs log flusher",
+	Journal* journal = (Journal*)_journal;
-		B_NORMAL_PRIORITY, _journal);
+	release_sem(journal->fLogFlusherSem);
-	if (id > 0)
-		resume_thread(id);
 }
 
 
 /*static*/ status_t
-Journal::_FlushLog(void* _journal)
+Journal::_LogFlusher(void* _journal)
 {
 	Journal* journal = (Journal*)_journal;
-	return journal->_FlushLog(false, false);
+	while (journal->fLogFlusherSem >= 0) {
+		if (acquire_sem(journal->fLogFlusherSem) != B_OK)
+			continue;
+
+		journal->_FlushLog(false, false);
+	}
+	return B_OK;
 }
 
 

src/add-ons/kernel/file_systems/bfs/Journal.h

@@ -60,7 +60,7 @@ private:
 								int32 event, void* _logEntry);
 	static	void			_TransactionIdle(int32 transactionID, int32 event,
 								void* _journal);
-	static	status_t		_FlushLog(void* _journal);
+	static	status_t		_LogFlusher(void* _journal);
 
 private:
 			Volume*			fVolume;
@@ -76,6 +76,9 @@ private:
 			int32			fTransactionID;
 			bool			fHasSubtransaction;
 			bool			fSeparateSubTransactions;
+
+			thread_id		fLogFlusher;
+			sem_id			fLogFlusherSem;
 };