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

Dmitry Panov dmitry.panov at yahoo.co.uk
Wed Dec 12 23:14:09 UTC 2012

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: 
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

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