From: Mike Snitzer <[email-masked]>

When specifying the feature 'default_hw_handler' multipath will use
the currently attached hardware handler instead of trying to attach the
one specified during table load.  If no hardware handler is attached the
specified hardware handler will be used.

Leverages scsi_dh_attach's ability to increment the scsi_dh's reference
count if the same scsi_dh name is provided when attaching -- currently
attached scsi_dh name is determined with scsi_dh_attached_handler_name.

Signed-off-by: Mike Snitzer <[email-masked]>
Tested-by: Babu Moger <[email-masked]>
Cc: Hannes Reinecke <[email-masked]>
Cc: Chandra Seetharaman <[email-masked]>
Reviewed-by: Chandra Seetharaman <[email-masked]>

---
 drivers/md/dm-mpath.c |   28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

Index: linux-3.4/drivers/md/dm-mpath.c
===================================================================
--- linux-3.4.orig/drivers/md/dm-mpath.c
+++ linux-3.4/drivers/md/dm-mpath.c
@@ -85,6 +85,7 @@ struct multipath {
 	unsigned queue_io:1;		/* Must we queue all I/O? */
 	unsigned queue_if_no_path:1;	/* Queue I/O if last path fails? */
 	unsigned saved_queue_if_no_path:1; /* Saved state during suspension */
+	unsigned use_default_hw_handler:1; /* Use attached device handler */
 
 	unsigned pg_init_retries;	/* Number of times to retry pg_init */
 	unsigned pg_init_count;		/* Number of times pg_init called */
@@ -568,6 +569,7 @@ static struct pgpath *parse_path(struct 
 	int r;
 	struct pgpath *p;
 	struct multipath *m = ti->private;
+	struct request_queue *q = NULL;
 
 	/* we need at least a path arg */
 	if (as->argc < 1) {
@@ -586,9 +588,19 @@ static struct pgpath *parse_path(struct 
 		goto bad;
 	}
 
-	if (m->hw_handler_name) {
-		struct request_queue *q = bdev_get_queue(p->path.dev->bdev);
+	if (m->use_default_hw_handler || m->hw_handler_name)
+		q = bdev_get_queue(p->path.dev->bdev);
+
+	if (m->use_default_hw_handler) {
+		const char *attached_handler_name =
+			scsi_dh_attached_handler_name(q, GFP_KERNEL);
+		if (attached_handler_name) {
+			kfree(m->hw_handler_name);
+			m->hw_handler_name = attached_handler_name;
+		}
+	}
 
+	if (m->hw_handler_name) {
 		r = scsi_dh_attach(q, m->hw_handler_name);
 		if (r == -EBUSY) {
 			/*
@@ -760,7 +772,7 @@ static int parse_features(struct dm_arg_
 	const char *arg_name;
 
 	static struct dm_arg _args[] = {
-		{0, 5, "invalid number of feature args"},
+		{0, 6, "invalid number of feature args"},
 		{1, 50, "pg_init_retries must be between 1 and 50"},
 		{0, 60000, "pg_init_delay_msecs must be between 0 and 60000"},
 	};
@@ -781,6 +793,11 @@ static int parse_features(struct dm_arg_
 			continue;
 		}
 
+		if (!strcasecmp(arg_name, "default_hw_handler")) {
+			m->use_default_hw_handler = 1;
+			continue;
+		}
+
 		if (!strcasecmp(arg_name, "pg_init_retries") &&
 		    (argc >= 1)) {
 			r = dm_read_arg(_args + 1, as, &m->pg_init_retries, &ti->error);
@@ -1364,13 +1381,16 @@ static int multipath_status(struct dm_ta
 	else {
 		DMEMIT("%u ", m->queue_if_no_path +
 			      (m->pg_init_retries > 0) * 2 +
-			      (m->pg_init_delay_msecs != DM_PG_INIT_DELAY_DEFAULT) * 2);
+			      (m->pg_init_delay_msecs != DM_PG_INIT_DELAY_DEFAULT) * 2 +
+			      m->use_default_hw_handler);
 		if (m->queue_if_no_path)
 			DMEMIT("queue_if_no_path ");
 		if (m->pg_init_retries)
 			DMEMIT("pg_init_retries %u ", m->pg_init_retries);
 		if (m->pg_init_delay_msecs != DM_PG_INIT_DELAY_DEFAULT)
 			DMEMIT("pg_init_delay_msecs %u ", m->pg_init_delay_msecs);
+		if (m->use_default_hw_handler)
+			DMEMIT("default_hw_handler ");
 	}
 
 	if (!m->hw_handler_name || type == STATUSTYPE_INFO)