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



Dne 13.12.2012 00:14, Dmitry Panov napsal(a):
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.



Hmmm this rather looks like a logical problem either in
the if() expression in (select_status == 0) branch,
or somehow 'magical' gulm fix applied in 2005 for add_to_lvmqueue()
should be running not just when message arrives.

Both patches seems to be not fixing the bug, but rather trying to go around broken logic in the main loop - it will need some thinking.

Zdenek


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