[PATCH] iscsi: make mutex for target scanning and unbinding per-session

[Chris Leech <cleech@redhat.com>]

Currently the iSCSI transport class synchronises target scanning and
unbinding with a host level mutex.  For multi-session hosts (offloading
iSCSI HBAs) connecting to storage arrays that may implement one
target-per-lun, this can result in the target scan work for hundreds of
sessions being serialized behind a single mutex.  With slow enough
response times, this can cause scan requests initiated from userspace to
block on the mutex long enough to trigger 120 sec hung task warnings.

I can't see any reason not to move this to a session level mutex and let
the target scans run in parallel, speeding up connecting to a large
number of targets.  Note that as iscsi_tcp creates a virtual host for
each session, software iSCSI is effectively doing this already.

Signed-off-by: Chris Leech <cleech@redhat.com>
---
 drivers/scsi/scsi_transport_iscsi.c | 19 ++++++-------------
 include/scsi/scsi_transport_iscsi.h |  2 +-
 2 files changed, 7 insertions(+), 14 deletions(-)

diff --git a/drivers/scsi/scsi_transport_iscsi.c b/drivers/scsi/scsi_transport_iscsi.c
index e4b3d8f..9daeb7f 100644
--- a/drivers/scsi/scsi_transport_iscsi.c
+++ b/drivers/scsi/scsi_transport_iscsi.c
@@ -1569,7 +1569,6 @@ static int iscsi_setup_host(struct transport_container *tc, struct device *dev,
 
 	memset(ihost, 0, sizeof(*ihost));
 	atomic_set(&ihost->nr_scans, 0);
-	mutex_init(&ihost->mutex);
 
 	iscsi_bsg_host_add(shost, ihost);
 	/* ignore any bsg add error - we just can't do sgio */
@@ -1789,8 +1788,6 @@ static int iscsi_user_scan_session(struct device *dev, void *data)
 {
 	struct iscsi_scan_data *scan_data = data;
 	struct iscsi_cls_session *session;
-	struct Scsi_Host *shost;
-	struct iscsi_cls_host *ihost;
 	unsigned long flags;
 	unsigned int id;
 
@@ -1801,10 +1798,7 @@ static int iscsi_user_scan_session(struct device *dev, void *data)
 
 	ISCSI_DBG_TRANS_SESSION(session, "Scanning session\n");
 
-	shost = iscsi_session_to_shost(session);
-	ihost = shost->shost_data;
-
-	mutex_lock(&ihost->mutex);
+	mutex_lock(&session->mutex);
 	spin_lock_irqsave(&session->lock, flags);
 	if (session->state != ISCSI_SESSION_LOGGED_IN) {
 		spin_unlock_irqrestore(&session->lock, flags);
@@ -1823,7 +1817,7 @@ static int iscsi_user_scan_session(struct device *dev, void *data)
 	}
 
 user_scan_exit:
-	mutex_unlock(&ihost->mutex);
+	mutex_unlock(&session->mutex);
 	ISCSI_DBG_TRANS_SESSION(session, "Completed session scan\n");
 	return 0;
 }
@@ -1999,26 +1993,24 @@ static void __iscsi_unbind_session(struct work_struct *work)
 	struct iscsi_cls_session *session =
 			container_of(work, struct iscsi_cls_session,
 				     unbind_work);
-	struct Scsi_Host *shost = iscsi_session_to_shost(session);
-	struct iscsi_cls_host *ihost = shost->shost_data;
 	unsigned long flags;
 	unsigned int target_id;
 
 	ISCSI_DBG_TRANS_SESSION(session, "Unbinding session\n");
 
 	/* Prevent new scans and make sure scanning is not in progress */
-	mutex_lock(&ihost->mutex);
+	mutex_lock(&session->mutex);
 	spin_lock_irqsave(&session->lock, flags);
 	if (session->target_id == ISCSI_MAX_TARGET) {
 		spin_unlock_irqrestore(&session->lock, flags);
-		mutex_unlock(&ihost->mutex);
+		mutex_unlock(&session->mutex);
 		return;
 	}
 
 	target_id = session->target_id;
 	session->target_id = ISCSI_MAX_TARGET;
 	spin_unlock_irqrestore(&session->lock, flags);
-	mutex_unlock(&ihost->mutex);
+	mutex_unlock(&session->mutex);
 
 	if (session->ida_used)
 		ida_simple_remove(&iscsi_sess_ida, target_id);
@@ -2051,6 +2043,7 @@ iscsi_alloc_session(struct Scsi_Host *shost, struct iscsi_transport *transport,
 	INIT_WORK(&session->unbind_work, __iscsi_unbind_session);
 	INIT_WORK(&session->scan_work, iscsi_scan_session);
 	spin_lock_init(&session->lock);
+	mutex_init(&session->mutex);
 
 	/* this is released in the dev's release function */
 	scsi_host_get(shost);
diff --git a/include/scsi/scsi_transport_iscsi.h b/include/scsi/scsi_transport_iscsi.h
index 6183d20..acf9d9d 100644
--- a/include/scsi/scsi_transport_iscsi.h
+++ b/include/scsi/scsi_transport_iscsi.h
@@ -238,6 +238,7 @@ struct iscsi_cls_session {
 	struct work_struct unblock_work;
 	struct work_struct scan_work;
 	struct work_struct unbind_work;
+	struct mutex mutex;
 
 	/* recovery fields */
 	int recovery_tmo;
@@ -272,7 +273,6 @@ struct iscsi_cls_session {
 
 struct iscsi_cls_host {
 	atomic_t nr_scans;
-	struct mutex mutex;
 	struct request_queue *bsg_q;
 	uint32_t port_speed;
 	uint32_t port_state;
-- 
2.1.0