storvsc: force discovery of LUNs that may have been removed.

The host asks the guest to scan when a LUN is removed or added.
The only way a guest can identify the removed LUN is when an I/O is
attempted on a removed LUN - the SRB status code indicates that the LUN
is invalid. We currently handle this SRB status and remove the device.

Rather than waiting for an I/O to remove the device, force the discovery of
LUNs that may have been removed prior to discovering LUNs that may have
been added.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/scsi/storvsc_drv.c

@@ -414,10 +414,36 @@ static void storvsc_host_scan(struct work_struct *work)
 {
 	struct storvsc_scan_work *wrk;
 	struct Scsi_Host *host;
+	struct scsi_device *sdev;
+	unsigned long flags;
 
 	wrk = container_of(work, struct storvsc_scan_work, work);
 	host = wrk->host;
 
+	/*
+	 * Before scanning the host, first check to see if any of the
+	 * currrently known devices have been hot removed. We issue a
+	 * "unit ready" command against all currently known devices.
+	 * This I/O will result in an error for devices that have been
+	 * removed. As part of handling the I/O error, we remove the device.
+	 *
+	 * When a LUN is added or removed, the host sends us a signal to
+	 * scan the host. Thus we are forced to discover the LUNs that
+	 * may have been removed this way.
+	 */
+	mutex_lock(&host->scan_mutex);
+	spin_lock_irqsave(host->host_lock, flags);
+	list_for_each_entry(sdev, &host->__devices, siblings) {
+		spin_unlock_irqrestore(host->host_lock, flags);
+		scsi_test_unit_ready(sdev, 1, 1, NULL);
+		spin_lock_irqsave(host->host_lock, flags);
+		continue;
+	}
+	spin_unlock_irqrestore(host->host_lock, flags);
+	mutex_unlock(&host->scan_mutex);
+	/*
+	 * Now scan the host to discover LUNs that may have been added.
+	 */
 	scsi_scan_host(host);
 
 	kfree(wrk);