isci: atomic device lookup and reference counting

We have unsafe references to remote devices that are notified to
disappear at lldd_dev_gone.  In order to clean this up we need a single
canonical source for device lookups and stable references once a lookup
succeeds.  Towards that end guarantee that domain_device.lldd_dev is
NULL as soon as we start the process of stopping a device.  Any code
path that wants to safely lookup a remote device must do so through
task->dev->lldd_dev (isci_lookup_device()).

For in-flight references outside of scic_lock we need reference counting
to ensure that the device is not recycled before we are done with it.
Simplify device back references to just scic_sds_request.target_device
which is now the only permissible internal reference that is maintained
relative to the reference count.

There were two occasions where we wanted new i/o's to be treated as
SAS_TASK_UNDELIVERED but where the domain_dev->lldd_dev link is still
intact.  Introduce a 'gone' flag to prevent i/o while waiting for libsas
to take action on the port down event.

One 'core' leftover is that we currently call
scic_remote_device_destruct() from isci_remote_device_deconstruct()
which is called when the 'core' says the device is stopped.  It would be
more natural for the final put to trigger
isci_remote_device_deconstruct() but this implementation is deferred as
it requires other changes.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/drivers/scsi/isci/remote_device.h b/drivers/scsi/isci/remote_device.h
index 2b6a5bb7bd6..05842b5f1e3 100644
--- a/drivers/scsi/isci/remote_device.h
+++ b/drivers/scsi/isci/remote_device.h
@@ -56,6 +56,7 @@
 #ifndef _ISCI_REMOTE_DEVICE_H_
 #define _ISCI_REMOTE_DEVICE_H_
 #include <scsi/libsas.h>
+#include <linux/kref.h>
 #include "scu_remote_node_context.h"
 #include "remote_node_context.h"
 #include "port.h"
@@ -134,7 +135,9 @@ struct isci_remote_device {
 	#define IDEV_STOP_PENDING 1
 	#define IDEV_ALLOCATED 2
 	#define IDEV_EH 3
+	#define IDEV_GONE 4
 	unsigned long flags;
+	struct kref kref;
 	struct isci_port *isci_port;
 	struct domain_device *domain_dev;
 	struct list_head node;
@@ -145,6 +148,26 @@ struct isci_remote_device {
 
 #define ISCI_REMOTE_DEVICE_START_TIMEOUT 5000
 
+/* device reference routines must be called under scic_lock */
+static inline struct isci_remote_device *isci_lookup_device(struct domain_device *dev)
+{
+	struct isci_remote_device *idev = dev->lldd_dev;
+
+	if (idev && !test_bit(IDEV_GONE, &idev->flags)) {
+		kref_get(&idev->kref);
+		return idev;
+	}
+
+	return NULL;
+}
+
+void isci_remote_device_release(struct kref *kref);
+static inline void isci_put_device(struct isci_remote_device *idev)
+{
+	if (idev)
+		kref_put(&idev->kref, isci_remote_device_release);
+}
+
 enum sci_status isci_remote_device_stop(struct isci_host *ihost,
 					struct isci_remote_device *idev);
 void isci_remote_device_nuke_requests(struct isci_host *ihost,