scsi: target/core: Use system workqueues for TMF

A quote from SAM-5: "The order in which task management requests are
processed is not specified by the SCSI architecture model.  The SCSI
architecture model does not require in-order delivery of such task
management requests or processing by the task manager in the order
received. To guarantee the processing order of task management requests
referencing sent to a specific logical unit, an application client should
not have more than one such task management request pending to that logical
unit." This means that it is safe to use the system workqueues instead of
tmr_wq for processing TMFs. An intended side effect of this patch is that
it enables concurrent processing of TMFs.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Mike Christie <mchristi@redhat.com>
Cc: David Disseldorp <ddiss@suse.de>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_device.c

@@ -986,18 +986,6 @@ int target_configure_device(struct se_device *dev)
 	if (ret)
 		goto out_destroy_device;
 
-	/*
-	 * Startup the struct se_device processing thread
-	 */
-	dev->tmr_wq = alloc_workqueue("tmr-%s", WQ_MEM_RECLAIM | WQ_UNBOUND, 1,
-				      dev->transport->name);
-	if (!dev->tmr_wq) {
-		pr_err("Unable to create tmr workqueue for %s\n",
-			dev->transport->name);
-		ret = -ENOMEM;
-		goto out_free_alua;
-	}
-
 	/*
 	 * Setup work_queue for QUEUE_FULL
 	 */
@@ -1026,8 +1014,6 @@ int target_configure_device(struct se_device *dev)
 
 	return 0;
 
-out_free_alua:
-	core_alua_free_lu_gp_mem(dev);
 out_destroy_device:
 	dev->transport->destroy_device(dev);
 out_free_index:
@@ -1046,8 +1032,6 @@ void target_free_device(struct se_device *dev)
 	WARN_ON(!list_empty(&dev->dev_sep_list));
 
 	if (target_dev_configured(dev)) {
-		destroy_workqueue(dev->tmr_wq);
-
 		dev->transport->destroy_device(dev);
 
 		mutex_lock(&device_mutex);

drivers/target/target_core_transport.c

@@ -3412,7 +3412,7 @@ int transport_generic_handle_tmr(
 	}
 
 	INIT_WORK(&cmd->work, target_tmr_work);
-	queue_work(cmd->se_dev->tmr_wq, &cmd->work);
+	schedule_work(&cmd->work);
 	return 0;
 }
 EXPORT_SYMBOL(transport_generic_handle_tmr);

include/target/target_core_base.h

@@ -796,7 +796,6 @@ struct se_device {
 	struct t10_pr_registration *dev_pr_res_holder;
 	struct list_head	dev_sep_list;
 	struct list_head	dev_tmr_list;
-	struct workqueue_struct *tmr_wq;
 	struct work_struct	qf_work_queue;
 	struct list_head	delayed_cmd_list;
 	struct list_head	state_list;