[RFC v2 4/6] scsi: improve scsi_device_lookup

[Douglas Gilbert <dgilbert@interlog.com>]

When the __scsi_device_lookup() function is given a "ctl" (i.e. the
latter part of "hctl" tuple), improve the loop to find a matching
device (LU) in the given host. Rather than loop over all LUs in the
host, first loop over all targets to find a match on "ct" then, if
found, loop over all LUs in that target for a match on "l". These
nested loops are better since they don't visit LUs belonging to
non-matching targets. This improvement flows through to the locked
version of this function, namely scsi_device_lookup().

Remove a 21 year old comment by the author that no longer seem
relevant.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
---
 drivers/scsi/scsi.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 0fb650aebcfb..9e7658aebdb7 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -35,7 +35,6 @@
  *
  *  Jiffies wrap fixes (host->resetting), 3 Dec 1998 Andrea Arcangeli
  *
- *  out_of_space hacks, D. Gilbert (dpg) 990608
  */
 
 #include <linux/module.h>
@@ -789,16 +788,31 @@ EXPORT_SYMBOL(scsi_device_lookup_by_target);
 struct scsi_device *__scsi_device_lookup(struct Scsi_Host *shost,
 		uint channel, uint id, u64 lun)
 {
-	unsigned long l_idx;
+	unsigned long l_idx, m_idx;
+	struct scsi_target *starg;
 	struct scsi_device *sdev;
 
-	xa_for_each_marked(&shost->__devices, l_idx, sdev,
+	if (xa_empty(&shost->__devices))
+		return NULL;
+	if (xa_empty(&shost->__targets))
+		goto inconsistent;
+	xa_for_each(&shost->__targets, l_idx, starg) {
+		if (!(starg->id == id && starg->channel == channel))
+			continue;
+		xa_for_each_marked(&starg->devices, m_idx, sdev,
+				   SCSI_XA_NON_SDEV_DEL) {
+			if (sdev->lun == lun)
+				return sdev;
+		}
+	}
+	return NULL;
+inconsistent:
+	xa_for_each_marked(&shost->__devices, m_idx, sdev,
 			   SCSI_XA_NON_SDEV_DEL) {
-		if (sdev->channel == channel && sdev->id == id &&
-				sdev->lun ==lun)
+		if (sdev->id == id && sdev->channel == channel &&
+		    sdev->lun == lun)
 			return sdev;
 	}
-
 	return NULL;
 }
 EXPORT_SYMBOL(__scsi_device_lookup);
-- 
2.25.1