[PATCH] SCSI: Scale up REPORT_LUNS timeout on failure

[Brian King <brking@linux.vnet.ibm.com>]

This patch fixes an issue seen with an IBM 2145 (SVC) where, following an error
injection test which results in paths going offline, when they came
back online, the path would timeout the REPORT_LUNS issued during the
scan. This timeout situation continued until retries were expired, resulting in
falling back to a sequential LUN scan. Then, since the target responds
with PQ=1, PDT=0 for all possible LUNs, due to the way the sequential
LUN scan code works, we end up adding 512 LUNs for each target, when there
is really only a small handful of LUNs that are actually present.

This patch doubles the timeout used on the REPORT_LUNS for each retry
after a timeout is seen on a REPORT_LUNS. This patch solves the issue
of 512 non existent LUNs showing up after this event. Running the test
with this patch still showed that we were regularly hitting two timeouts,
but the third, and final, REPORT_LUNS was always successful.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
---

 drivers/scsi/scsi_scan.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff -puN drivers/scsi/scsi_scan.c~scsi_report_luns_timeout_escalate drivers/scsi/scsi_scan.c
--- linux/drivers/scsi/scsi_scan.c~scsi_report_luns_timeout_escalate	2015-09-02 08:49:07.268243497 -0500
+++ linux-bjking1/drivers/scsi/scsi_scan.c	2015-09-02 08:49:07.272243461 -0500
@@ -1304,6 +1304,7 @@ static int scsi_report_lun_scan(struct s
 	struct scsi_device *sdev;
 	struct Scsi_Host *shost = dev_to_shost(&starget->dev);
 	int ret = 0;
+	int timeout = SCSI_TIMEOUT + 4 * HZ;
 
 	/*
 	 * Only support SCSI-3 and up devices if BLIST_NOREPORTLUN is not set.
@@ -1383,7 +1384,7 @@ retry:
 
 		result = scsi_execute_req(sdev, scsi_cmd, DMA_FROM_DEVICE,
 					  lun_data, length, &sshdr,
-					  SCSI_TIMEOUT + 4 * HZ, 3, NULL);
+					  timeout, 3, NULL);
 
 		SCSI_LOG_SCAN_BUS(3, sdev_printk (KERN_INFO, sdev,
 				"scsi scan: REPORT LUNS"
@@ -1392,6 +1393,8 @@ retry:
 				retries, result));
 		if (result == 0)
 			break;
+		else if (host_byte(result) == DID_TIME_OUT)
+			timeout = timeout * 2;
 		else if (scsi_sense_valid(&sshdr)) {
 			if (sshdr.sense_key != UNIT_ATTENTION)
 				break;
_