ocfs2: ocfs2_downconvert_lock failure results in deadlock

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.

Link: https://lkml.kernel.org/r/20210830044621.12544-1-ghe@suse.com
Signed-off-by: Gang He <ghe@suse.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Gang He <ghe@suse.com>
Cc: Jun Piao <piaojun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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 @@ downconvert:
 	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)