scsi: Use ida for host number management

Update the SCSI hosts module to use ida to manage its host_no index
instead of an atomic integer. This means that the SCSI host number will
now be reclaimable.

Use the ida "simple" mechanism, since there should be no need for a
separate spin lock current usage. Ida was chosen over idr because the
hosts module already has its own instance and locking mechanisms that
aren't easily changed.

Reviewed-by: Hannes Reinecke <[e-mail]>
Reviewed-by: Johannes Thumshirn <[e-mail]>
Signed-off-by: Lee Duncan <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/hosts.c

···
 #include <linux/transport_class.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
-
+#include <linux/idr.h>
 #include <scsi/scsi_device.h>
 #include <scsi/scsi_host.h>
 #include <scsi/scsi_transport.h>
···
 #include "scsi_logging.h"
 
 
-static atomic_t scsi_host_next_hn = ATOMIC_INIT(0);	/* host_no for next new host */
+static DEFINE_IDA(host_index_ida);
 
 
 static void scsi_host_cls_release(struct device *dev)
···
 
 	kfree(shost->shost_data);
 
+	ida_simple_remove(&host_index_ida, shost->host_no);
+
 	if (parent)
 		put_device(parent);
 	kfree(shost);
···
 {
 	struct Scsi_Host *shost;
 	gfp_t gfp_mask = GFP_KERNEL;
+	int index;
 
 	if (sht->unchecked_isa_dma && privsize)
 		gfp_mask |= __GFP_DMA;
···
 	init_waitqueue_head(&shost->host_wait);
 	mutex_init(&shost->scan_mutex);
 
-	/*
-	 * subtract one because we increment first then return, but we need to
-	 * know what the next host number was before increment
-	 */
-	shost->host_no = atomic_inc_return(&scsi_host_next_hn) - 1;
+	index = ida_simple_get(&host_index_ida, 0, 0, GFP_KERNEL);
+	if (index < 0)
+		goto fail_kfree;
+	shost->host_no = index;
+
 	shost->dma_channel = 0xff;
 
 	/* These three are default values which can be overridden */
···
 		shost_printk(KERN_WARNING, shost,
 			"error handler thread failed to spawn, error = %ld\n",
 			PTR_ERR(shost->ehandler));
-		goto fail_kfree;
+		goto fail_index_remove;
 	}
 
 	shost->tmf_work_q = alloc_workqueue("scsi_tmf_%d",
···
 
  fail_kthread:
 	kthread_stop(shost->ehandler);
+ fail_index_remove:
+	ida_simple_remove(&host_index_ida, shost->host_no);
  fail_kfree:
 	kfree(shost);
 	return NULL;
···
 void scsi_exit_hosts(void)
 {
 	class_unregister(&shost_class);
+	ida_destroy(&host_index_ida);
 }
 
 int scsi_is_host_device(const struct device *dev)