target: Add transport_handle_cdb_direct optimization

This patch adds a transport_handle_cdb_direct() optimization for mapping
and queueing tasks directly from within fabric processing context by calling
the newly exported transport_generic_new_cmd().  This currently expects to
be called from process context only, and will fail if called within interrupt
context.

This patch also leaves transport_generic_handle_cdb() unmodified for the
moment to function as expected with existing tcm_fc and ib_srpt fabrics,
and will be removed once these have been converted and tested with v4.1
code using transport_handle_cdb_direct().

Based on Andy's original patch here:

[PATCH 39/42] target: Call transport_new_cmd instead of adding to cmd queue
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_transport.c

@@ -1842,11 +1842,35 @@ int transport_generic_handle_cdb(
 		printk(KERN_ERR "cmd->se_lun is NULL\n");
 		return -EINVAL;
 	}
+
 	transport_add_cmd_to_queue(cmd, TRANSPORT_NEW_CMD);
 	return 0;
 }
 EXPORT_SYMBOL(transport_generic_handle_cdb);
 
+/*
+ * Used by fabric module frontends to queue tasks directly.
+ * Many only be used from process context only
+ */
+int transport_handle_cdb_direct(
+	struct se_cmd *cmd)
+{
+	if (!cmd->se_lun) {
+		dump_stack();
+		printk(KERN_ERR "cmd->se_lun is NULL\n");
+		return -EINVAL;
+	}
+	if (in_interrupt()) {
+		dump_stack();
+		printk(KERN_ERR "transport_generic_handle_cdb cannot be called"
+				" from interrupt context\n");
+		return -EINVAL;
+	}
+
+	return transport_generic_new_cmd(cmd);
+}
+EXPORT_SYMBOL(transport_handle_cdb_direct);
+
 /*
  * Used by fabric module frontends defining a TFO->new_cmd_map() caller
  * to  queue up a newly setup se_cmd w/ TRANSPORT_NEW_CMD_MAP in order to

include/target/target_core_transport.h

@@ -166,6 +166,7 @@ extern void transport_init_se_cmd(struct se_cmd *,
 extern void transport_free_se_cmd(struct se_cmd *);
 extern int transport_generic_allocate_tasks(struct se_cmd *, unsigned char *);
 extern int transport_generic_handle_cdb(struct se_cmd *);
+extern int transport_handle_cdb_direct(struct se_cmd *);
 extern int transport_generic_handle_cdb_map(struct se_cmd *);
 extern int transport_generic_handle_data(struct se_cmd *);
 extern void transport_new_cmd_failure(struct se_cmd *);