md-cluster: fix deadlock issue on message lock

There is problem with previous communication mechanism, and we got below
deadlock scenario with cluster which has 3 nodes.

	Sender                	    Receiver        		Receiver

	token(EX)
       message(EX)
      writes message
   downconverts message(CR)
      requests ack(EX)
		                  get message(CR)            gets message(CR)
                		  reads message                reads message
		               requests EX on message    requests EX on message

To fix this problem, we do the following changes:

1. the sender downconverts MESSAGE to CW rather than CR.
2. and the receiver request PR lock not EX lock on message.

And in case we failed to down-convert EX to CW on message, it is better to
unlock message otherthan still hold the lock.

Reviewed-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Lidong Zhong <ldzhong@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/Documentation/md-cluster.txt b/Documentation/md-cluster.txt
index de1af7db3355..1b794369e03a 100644
--- a/Documentation/md-cluster.txt
+++ b/Documentation/md-cluster.txt
@@ -91,7 +91,7 @@ The algorithm is:
     this message inappropriate or redundant.
 
  3. sender write LVB.
-    sender down-convert MESSAGE from EX to CR
+    sender down-convert MESSAGE from EX to CW
     sender try to get EX of ACK
     [ wait until all receiver has *processed* the MESSAGE ]
 
@@ -112,7 +112,7 @@ The algorithm is:
     sender down-convert ACK from EX to CR
     sender release MESSAGE
     sender release TOKEN
-                               receiver upconvert to EX of MESSAGE
+                               receiver upconvert to PR of MESSAGE
                                receiver get CR of ACK
                                receiver release MESSAGE