[PATCH v2] ocfs2: ocfs2_downconvert_lock failure results in deadlock

[Gang He]

Usually, ocfs2_downconvert_lock() function always downconverts
dlm lock to the expected level for satisfy dlm bast requests
from the other nodes.
But there is a rare situation. When dlm lock conversion is being
canceled, ocfs2_downconvert_lock() function will return -EBUSY.
You need to be aware that ocfs2_cancel_convert() function is
asynchronous in fsdlm implementation.
If we does not requeue this lockres entry, ocfs2 downconvert
thread no longer handles this dlm lock bast request. Then, the
other nodes will not get the dlm lock again, the current node's
process will be blocked when acquire this dlm lock again.

Signed-off-by: Gang He <ghe@xxxxxxxx>
---
v2: keep error print in ocfs2_downconvert_lock function, add more detailed
comments in code, add msleep to avoid repeated attempts. 
---
 fs/ocfs2/dlmglue.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
index 48fd369c29a4..f8f561850470 100644
--- a/fs/ocfs2/dlmglue.c
+++ b/fs/ocfs2/dlmglue.c
@@ -16,6 +16,7 @@
 #include <linux/debugfs.h>
 #include <linux/seq_file.h>
 #include <linux/time.h>
+#include <linux/delay.h>
 #include <linux/quotaops.h>
 #include <linux/sched/signal.h>
 
@@ -3912,6 +3913,17 @@ static int ocfs2_unblock_lock(struct ocfs2_super *osb,
 	spin_unlock_irqrestore(&lockres->l_lock, flags);
 	ret = ocfs2_downconvert_lock(osb, lockres, new_level, set_lvb,
 				     gen);
+	/* The dlm lock convert is being cancelled in background,
+	 * ocfs2_cancel_convert() is asynchronous in fs/dlm,
+	 * requeue it, try again later.
+	 */
+	if (ret == -EBUSY) {
+		ctl->requeue = 1;
+		mlog(ML_BASTS, "lockres %s, ReQ: Downconvert busy\n",
+		     lockres->l_name);
+		ret = 0;
+		msleep(20);
+	}
 
 leave:
 	if (ret)
-- 
2.12.3