NFS hangs in __ocfs2_cluster_lock due to race with ocfs2_unblock_lock

NFS hangs in __ocfs2_cluster_lock due to race with ocfs2_unblock_lock

Orabug: 20933419

NFS on a 2 node ocfs2 cluster each node exporting dir. The lock causing
the hang is the global bit map inode lock.  Node 1 is master, has
the lock granted in PR mode; Node 2 is in the converting list (PR ->
EX). There are no holders of the lock on the master node so it should
downconvert to NL and grant EX to node 2 but that does not happen.
BLOCKED + QUEUED in lock res are set and it is on osb blocked list.
Threads are waiting in __ocfs2_cluster_lock on BLOCKED.  One thread wants
EX, rest want PR. So it is as though the downconvert thread needs to be
kicked to complete the conv.

The hang is caused by an EX req coming into  __ocfs2_cluster_lock on
the heels of a PR req after it sets BUSY (drops l_lock, releasing EX
thread), forcing the incoming EX to wait on BUSY without doing anything.
PR has called ocfs2_dlm_lock, which  sets the node 1 lock from NL ->
PR, queues ast.

At this time, upconvert (PR ->EX) arrives from node 2, finds conflict with
node 1 lock in PR, so the lock res is put on dlm thread's dirty listt.

After ret from ocf2_dlm_lock, PR thread now waits behind EX on BUSY till
awoken by ast.

Now it is dlm_thread that serially runs dlm_shuffle_lists, ast,  bast,
in that order.  dlm_shuffle_lists ques a bast on behalf of node 2
(which will be run by dlm_thread right after the ast).  ast does its
part, sets UPCONVERT_FINISHING, clears BUSY and wakes its waiters. Next,
dlm_thread runs  bast. It sets BLOCKED and kicks dc thread.  dc thread
runs ocfs2_unblock_lock, but since UPCONVERT_FINISHING set, skips doing
anything and reques.

Inside of __ocfs2_cluster_lock, since EX has been waiting on BUSY ahead
of PR, it wakes up first, finds BLOCKED set and skips doing anything
but clearing UPCONVERT_FINISHING (which was actually "meant" for the
PR thread), and this time waits on BLOCKED.  Next, the PR thread comes
out of wait but since UPCONVERT_FINISHING is not set, it skips updating
the l_ro_holders and goes straight to wait on BLOCKED. So there, we
have a hang! Threads in __ocfs2_cluster_lock wait on BLOCKED, lock
res in osb blocked list. Only when dc thread is awoken, it will run
ocfs2_unblock_lock and things will unhang.

One way to fix this is to wake the dc thread on the flag after clearing
UPCONVERT_FINISHING

Signed-off-by: Tariq Saeed <tariq.x.saeed@oracle.com>
Reviewed-by: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>