sd: get disk reference in sd_check_events()

sd_check_events() is called asynchronously, and might race
with device removal. So always take a disk reference when
processing the event to avoid the device being removed while
the event is processed.
Signed-off-by: NHannes Reinecke <hare@suse.de>
Reviewed-by: NEwan D. Milne <emilne@redhat.com>
Reviewed-by: NBart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sd.c

@@ -1398,11 +1398,15 @@ static int media_not_present(struct scsi_disk *sdkp,
  **/
 static unsigned int sd_check_events(struct gendisk *disk, unsigned int clearing)
 {
-	struct scsi_disk *sdkp = scsi_disk(disk);
-	struct scsi_device *sdp = sdkp->device;
+	struct scsi_disk *sdkp = scsi_disk_get(disk);
+	struct scsi_device *sdp;
 	struct scsi_sense_hdr *sshdr = NULL;
 	int retval;
 
+	if (!sdkp)
+		return 0;
+
+	sdp = sdkp->device;
 	SCSI_LOG_HLQUEUE(3, sd_printk(KERN_INFO, sdkp, "sd_check_events\n"));
 
 	/*
@@ -1459,6 +1463,7 @@ static unsigned int sd_check_events(struct gendisk *disk, unsigned int clearing)
 	kfree(sshdr);
 	retval = sdp->changed ? DISK_EVENT_MEDIA_CHANGE : 0;
 	sdp->changed = 0;
+	scsi_disk_put(sdkp);
 	return retval;
 }