From d4492eea92d27e78f151e11bf6de49765d4abe5f Mon Sep 17 00:00:00 2001
From: dimitr <dimitr@users.sourceforge.net>
Date: Sun, 10 Aug 2014 20:11:41 +0000
Subject: [PATCH] Better (methinks) synchronization for the monitoring stuff.
 Being field tested.

---
 src/jrd/Attachment.cpp | 23 +++++++++++++----------
 src/jrd/Attachment.h   |  4 ++--
 src/jrd/Monitoring.cpp | 25 +++++++++++--------------
 3 files changed, 26 insertions(+), 26 deletions(-)

diff --git a/src/jrd/Attachment.cpp b/src/jrd/Attachment.cpp
index 1c4e98b4c8..d4ce3d58f0 100644
--- a/src/jrd/Attachment.cpp
+++ b/src/jrd/Attachment.cpp
@@ -434,7 +434,7 @@ void Jrd::Attachment::initLocks(thread_db* tdbb)
 		Lock(tdbb, sizeof(SLONG), LCK_monitor, this, blockingAstMonitor);
 	att_monitor_lock = lock;
 	lock->lck_key.lck_long = att_attachment_id;
-	LCK_lock(tdbb, lock, LCK_SR, LCK_WAIT);
+	LCK_lock(tdbb, lock, LCK_EX, LCK_WAIT);
 
 	// Unless we're a system attachment, allocate the cancellation lock
 
@@ -644,17 +644,20 @@ int Jrd::Attachment::blockingAstMonitor(void* ast_object)
 
 		AsyncContextHolder tdbb(dbb, FB_FUNCTION, attachment->att_monitor_lock);
 
-		try
+		if (!(attachment->att_flags & ATT_monitor_done))
 		{
-			Monitoring::dumpAttachment(tdbb, attachment, true);
-		}
-		catch (const Exception& ex)
-		{
-			iscLogException("Cannot dump the monitoring data", ex);
-		}
+			try
+			{
+				Monitoring::dumpAttachment(tdbb, attachment, true);
+			}
+			catch (const Exception& ex)
+			{
+				iscLogException("Cannot dump the monitoring data", ex);
+			}
 
-		LCK_release(tdbb, attachment->att_monitor_lock);
-		attachment->att_flags |= ATT_monitor_off;
+			LCK_downgrade(tdbb, attachment->att_monitor_lock);
+			attachment->att_flags |= ATT_monitor_done;
+		}
 	}
 	catch (const Exception&)
 	{} // no-op
diff --git a/src/jrd/Attachment.h b/src/jrd/Attachment.h
index 10268e7695..f24f228f22 100644
--- a/src/jrd/Attachment.h
+++ b/src/jrd/Attachment.h
@@ -478,8 +478,8 @@ const ULONG ATT_manual_lock			= 0x00800L;	// Was locked manually
 const ULONG ATT_async_manual_lock	= 0x01000L;	// Async mutex was locked manually
 const ULONG ATT_purge_started		= 0x02000L; // Purge already started - avoid 2 purges at once
 const ULONG ATT_system				= 0x04000L; // Special system attachment
-const ULONG ATT_creator				= 0x08000L; // This attachment created the DB.
-const ULONG ATT_monitor_off			= 0x10000L; // Monitoring lock is released
+const ULONG ATT_creator				= 0x08000L; // This attachment created the DB
+const ULONG ATT_monitor_done		= 0x10000L; // Monitoring data is refreshed
 
 const ULONG ATT_NO_CLEANUP			= (ATT_no_cleanup | ATT_notify_gc);
 
diff --git a/src/jrd/Monitoring.cpp b/src/jrd/Monitoring.cpp
index 380f641653..232b169b04 100644
--- a/src/jrd/Monitoring.cpp
+++ b/src/jrd/Monitoring.cpp
@@ -351,15 +351,16 @@ MonitoringSnapshot::MonitoringSnapshot(thread_db* tdbb, MemoryPool& pool)
 	RecordBuffer* const ctx_var_buffer = allocBuffer(tdbb, pool, rel_mon_ctx_vars);
 	RecordBuffer* const mem_usage_buffer = allocBuffer(tdbb, pool, rel_mon_mem_usage);
 
-	// Release our own lock
-
-	LCK_release(tdbb, attachment->att_monitor_lock);
-	attachment->att_flags &= ~ATT_monitor_off;
-
-	// Dump our own data
+	// Dump our own data and downgrade the lock, if required
 
 	Monitoring::dumpAttachment(tdbb, attachment, false);
 
+	if (!(attachment->att_flags & ATT_monitor_done))
+	{
+		LCK_convert(tdbb, attachment->att_monitor_lock, LCK_SR, LCK_NO_WAIT);
+		attachment->att_flags |= ATT_monitor_done;
+	}
+
 	// Enumerate active sessions
 
 	MonitoringData::SessionList sessions(pool);
@@ -384,7 +385,7 @@ MonitoringSnapshot::MonitoringSnapshot(thread_db* tdbb, MemoryPool& pool)
 			{
 				lock->lck_key.lck_long = *iter;
 
-				if (LCK_lock(tdbb, lock, LCK_EX, LCK_WAIT))
+				if (LCK_lock(tdbb, lock, LCK_SR, LCK_WAIT))
 					LCK_release(tdbb, lock);
 			}
 		}
@@ -419,10 +420,6 @@ MonitoringSnapshot::MonitoringSnapshot(thread_db* tdbb, MemoryPool& pool)
 		dbb->dbb_monitoring_data->read(self_att_id, temp_space);
 	}
 
-	// Mark our attachment as requesting a new lock
-
-	attachment->att_flags |= ATT_monitor_off;
-
 	string databaseName(dbb->dbb_database_name.c_str());
 	ISC_systemToUtf8(databaseName);
 
@@ -1148,11 +1145,11 @@ void Monitoring::checkState(thread_db* tdbb)
 
 	Jrd::Attachment* const attachment = tdbb->getAttachment();
 
-	if (attachment->att_flags & ATT_monitor_off)
+	if (attachment->att_flags & ATT_monitor_done)
 	{
 		// Enable signal handler for the monitoring stuff
-		attachment->att_flags &= ~ATT_monitor_off;
-		LCK_lock(tdbb, attachment->att_monitor_lock, LCK_SR, LCK_WAIT);
+		attachment->att_flags &= ~ATT_monitor_done;
+		LCK_convert(tdbb, attachment->att_monitor_lock, LCK_EX, LCK_WAIT);
 	}
 }