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[linux-lvm] clvmd on cman waits forever holding the P_#global lock on node re-join



Hi everyone,

I've been testing clvm recently and noticed that often the operations are blocked when a node rejoins the cluster after being fenced or power cycled. I've done some investigation and found a number of issues relating to clvm. Here is what's happening:


- When a node is fenced there is no "port closed" message sent to clvm which means the node id remains in the updown hash, although the node itself is removed from the nodes list after a "configuration changed" message is received.

- Then, when the node rejoins, another "configuration changed" message arrives but because the node id is still in the hash, it is assumed that clvmd on that node is running even though it might not be the case yet (in my case clvmd is a pacemaker resource so it takes a couple of seconds before it's started).

- This causes the expected_replies count set to a higher number than it should be, and as a result there are never enough replies received.

- There is a problem with handling of the cmd_timeout which appears to be fixed today (what a coincidence!) by this patch: https://www.redhat.com/archives/lvm-devel/2012-December/msg00024.html The reason why I was hitting this bug is because I'm using Linux Cluster Management Console which polls LVM often enough so that the timeout code never ran. I have fixed this independently and even though my efforts are now probably wasted I'm attaching a patch for your consideration. I believe it enforces the timeout more strictly.

Now, the questions:

1. If the problem with stuck entry in the updown hash is fixed it will cause operations to fail until clvmd is started on the re-joined node. Is there any particular reason for making them fail? Is it to avoid a race condition when newly started clvmd might not receive a message generated by an 'old' node?

2. The current expected_replies counter seems a bit flawed to me because it will fail if a node leaves the cluster before it sends a reply. Should it be handled differently? For example instead of a simple counter we could have a list of nodes which should be updated when a node leaves the cluster.


Best regards,

--
Dmitry Panov

--- clvmd.c.orig	2012-03-01 21:14:43.000000000 +0000
+++ clvmd.c	2012-12-12 22:57:27.917901181 +0000
@@ -838,30 +838,33 @@
 	sigaddset(&ss, SIGTERM);
 	pthread_sigmask(SIG_UNBLOCK, &ss, NULL);
 	/* Main loop */
+	time_t deadline = time(NULL) + cmd_timeout;
 	while (!quit) {
 		fd_set in;
 		int select_status;
 		struct local_client *thisfd;
-		struct timeval tv = { cmd_timeout, 0 };
-		int quorate = clops->is_quorate();
-
-		/* Wait on the cluster FD and all local sockets/pipes */
-		local_client_head.fd = clops->get_main_cluster_fd();
-		FD_ZERO(&in);
-		for (thisfd = &local_client_head; thisfd != NULL;
-		     thisfd = thisfd->next) {
-
-			if (thisfd->removeme)
-				continue;
-
-			/* if the cluster is not quorate then don't listen for new requests */
-			if ((thisfd->type != LOCAL_RENDEZVOUS &&
-			     thisfd->type != LOCAL_SOCK) || quorate)
-				FD_SET(thisfd->fd, &in);
+		struct timeval tv = { deadline - time(NULL), 0 };
+		if (tv.tv_sec > 0) {
+		    /* Wait on the cluster FD and all local sockets/pipes */
+		    int quorate = clops->is_quorate();
+		    local_client_head.fd = clops->get_main_cluster_fd();
+		    FD_ZERO(&in);
+		    for (thisfd = &local_client_head; thisfd != NULL;
+			 thisfd = thisfd->next) {
+
+			    if (thisfd->removeme)
+				    continue;
+
+			    /* if the cluster is not quorate then don't listen for new requests */
+			    if ((thisfd->type != LOCAL_RENDEZVOUS &&
+				 thisfd->type != LOCAL_SOCK) || quorate)
+				    FD_SET(thisfd->fd, &in);
+		    }
+
+		    select_status = select(FD_SETSIZE, &in, NULL, NULL, &tv);
+		} else {
+		    select_status = 0;
 		}
-
-		select_status = select(FD_SETSIZE, &in, NULL, NULL, &tv);
-
 		if (reread_config) {
 			int saved_errno = errno;
 
@@ -936,28 +939,34 @@
 			}
 		}
 
-		/* Select timed out. Check for clients that have been waiting too long for a response */
-		if (select_status == 0) {
+		/* Check for clients that have been waiting too long for a response and set a new wake-up deadline */
+		if (select_status >= 0) {
 			time_t the_time = time(NULL);
+			deadline = the_time + cmd_timeout;
 
 			for (thisfd = &local_client_head; thisfd != NULL;
 			     thisfd = thisfd->next) {
 				if (thisfd->type == LOCAL_SOCK
 				    && thisfd->bits.localsock.sent_out
-				    && thisfd->bits.localsock.sent_time +
-				    cmd_timeout < the_time
 				    && thisfd->bits.localsock.
 				    expected_replies !=
 				    thisfd->bits.localsock.num_replies) {
-					/* Send timed out message + replies we already have */
-					DEBUGLOG
-					    ("Request timed-out (send: %ld, now: %ld)\n",
-					     thisfd->bits.localsock.sent_time,
-					     the_time);
-
-					thisfd->bits.localsock.all_success = 0;
-
-					request_timed_out(thisfd);
+					time_t this_deadline = thisfd->bits.localsock.sent_time +
+					    cmd_timeout;
+					if (this_deadline < the_time) {
+						/* Send timed out message + replies we already have */
+						DEBUGLOG
+						    ("Request timed-out (send: %ld, now: %ld)\n",
+					    	thisfd->bits.localsock.sent_time,
+					    	the_time);
+
+					    thisfd->bits.localsock.all_success = 0;
+
+					    request_timed_out(thisfd);
+					} else {
+					    if (this_deadline < deadline)
+						deadline = this_deadline;
+					}
 				}
 			}
 		}

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