target: Add target_submit_cmd() for process context fabric submission

This patch adds a target_submit_cmd() caller that can be used by fabrics
to submit an uninitialized se_cmd descriptor to an struct se_session +
unpacked_lun from workqueue process context.  This call will invoke the
following steps:

- transport_init_se_cmd() to setup se_cmd specific pointers
- Obtain se_cmd->cmd_kref references with target_get_sess_cmd()
- set se_cmd->t_tasks_bidi
- transport_lookup_cmd_lun() to setup struct se_cmd->se_lun from
  the passed unpacked_lun
- transport_generic_allocate_tasks() to setup the passed *cdb, and
- transport_handle_cdb_direct() handle READ dispatch or WRITE
  ready-to-transfer callback to fabric

v2 changes from hch feedback:

*) Add target_sc_flags_table for target_submit_cmd flags
*) Rename bidi parameter to flags, add TARGET_SCF_BIDI_OP
*) Convert checks to BUG_ON
*) Add out_check_cond for transport_send_check_condition_and_sense
   usage

v3 changes:

*) Add TARGET_SCF_ACK_KREF for target_submit_cmd into
   target_get_sess_cmd to determine when the fabric caller is expecting
   a second kref_put() from fabric packet acknowledgement.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_transport.c

@@ -1642,6 +1642,80 @@ int transport_handle_cdb_direct(
 }
 EXPORT_SYMBOL(transport_handle_cdb_direct);
 
+/**
+ * target_submit_cmd - lookup unpacked lun and submit uninitialized se_cmd
+ *
+ * @se_cmd: command descriptor to submit
+ * @se_sess: associated se_sess for endpoint
+ * @cdb: pointer to SCSI CDB
+ * @sense: pointer to SCSI sense buffer
+ * @unpacked_lun: unpacked LUN to reference for struct se_lun
+ * @data_length: fabric expected data transfer length
+ * @task_addr: SAM task attribute
+ * @data_dir: DMA data direction
+ * @flags: flags for command submission from target_sc_flags_tables
+ *
+ * This may only be called from process context, and also currently
+ * assumes internal allocation of fabric payload buffer by target-core.
+ **/
+int target_submit_cmd(struct se_cmd *se_cmd, struct se_session *se_sess,
+		unsigned char *cdb, unsigned char *sense, u32 unpacked_lun,
+		u32 data_length, int task_attr, int data_dir, int flags)
+{
+	struct se_portal_group *se_tpg;
+	int rc;
+
+	se_tpg = se_sess->se_tpg;
+	BUG_ON(!se_tpg);
+	BUG_ON(se_cmd->se_tfo || se_cmd->se_sess);
+	BUG_ON(in_interrupt());
+	/*
+	 * Initialize se_cmd for target operation.  From this point
+	 * exceptions are handled by sending exception status via
+	 * target_core_fabric_ops->queue_status() callback
+	 */
+	transport_init_se_cmd(se_cmd, se_tpg->se_tpg_tfo, se_sess,
+				data_length, data_dir, task_attr, sense);
+	/*
+	 * Obtain struct se_cmd->cmd_kref reference and add new cmd to
+	 * se_sess->sess_cmd_list.  A second kref_get here is necessary
+	 * for fabrics using TARGET_SCF_ACK_KREF that expect a second
+	 * kref_put() to happen during fabric packet acknowledgement.
+	 */
+	target_get_sess_cmd(se_sess, se_cmd, (flags & TARGET_SCF_ACK_KREF));
+	/*
+	 * Signal bidirectional data payloads to target-core
+	 */
+	if (flags & TARGET_SCF_BIDI_OP)
+		se_cmd->se_cmd_flags |= SCF_BIDI;
+	/*
+	 * Locate se_lun pointer and attach it to struct se_cmd
+	 */
+	if (transport_lookup_cmd_lun(se_cmd, unpacked_lun) < 0)
+		goto out_check_cond;
+	/*
+	 * Sanitize CDBs via transport_generic_cmd_sequencer() and
+	 * allocate the necessary tasks to complete the received CDB+data
+	 */
+	rc = transport_generic_allocate_tasks(se_cmd, cdb);
+	if (rc != 0)
+		goto out_check_cond;
+	/*
+	 * Dispatch se_cmd descriptor to se_lun->lun_se_dev backend
+	 * for immediate execution of READs, otherwise wait for
+	 * transport_generic_handle_data() to be called for WRITEs
+	 * when fabric has filled the incoming buffer.
+	 */
+	transport_handle_cdb_direct(se_cmd);
+	return 0;
+
+out_check_cond:
+	transport_send_check_condition_and_sense(se_cmd,
+				se_cmd->scsi_sense_reason, 0);
+	return 0;
+}
+EXPORT_SYMBOL(target_submit_cmd);
+
 /*
  * 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
@@ -3910,13 +3984,21 @@ EXPORT_SYMBOL(transport_generic_free_cmd);
 /* target_get_sess_cmd - Add command to active ->sess_cmd_list
  * @se_sess:	session to reference
  * @se_cmd:	command descriptor to add
+ * @ack_kref:	Signal that fabric will perform an ack target_put_sess_cmd()
  */
-void target_get_sess_cmd(struct se_session *se_sess, struct se_cmd *se_cmd)
+void target_get_sess_cmd(struct se_session *se_sess, struct se_cmd *se_cmd,
+			bool ack_kref)
 {
 	unsigned long flags;
 
 	kref_init(&se_cmd->cmd_kref);
-	kref_get(&se_cmd->cmd_kref);
+	/*
+	 * Add a second kref if the fabric caller is expecting to handle
+	 * fabric acknowledgement that requires two target_put_sess_cmd()
+	 * invocations before se_cmd descriptor release.
+	 */
+	if (ack_kref == true)
+		kref_get(&se_cmd->cmd_kref);
 
 	spin_lock_irqsave(&se_sess->sess_cmd_lock, flags);
 	list_add_tail(&se_cmd->se_cmd_list, &se_sess->sess_cmd_list);

include/target/target_core_base.h

@@ -226,6 +226,11 @@ enum tcm_sense_reason_table {
 	TCM_RESERVATION_CONFLICT		= 0x10,
 };
 
+enum target_sc_flags_table {
+	TARGET_SCF_BIDI_OP		= 0x01,
+	TARGET_SCF_ACK_KREF		= 0x02,
+};
+
 /* fabric independent task management function values */
 enum tcm_tmreq_table {
 	TMR_ABORT_TASK		= 1,

include/target/target_core_fabric.h

@@ -118,6 +118,8 @@ void	transport_init_se_cmd(struct se_cmd *, struct target_core_fabric_ops *,
 		struct se_session *, u32, int, int, unsigned char *);
 int	transport_lookup_cmd_lun(struct se_cmd *, u32);
 int	transport_generic_allocate_tasks(struct se_cmd *, unsigned char *);
+int	target_submit_cmd(struct se_cmd *, struct se_session *, unsigned char *,
+		unsigned char *, u32, u32, int, int, int);
 int	transport_handle_cdb_direct(struct se_cmd *);
 int	transport_generic_handle_cdb_map(struct se_cmd *);
 int	transport_generic_handle_data(struct se_cmd *);
@@ -134,7 +136,7 @@ bool	transport_wait_for_tasks(struct se_cmd *);
 int	transport_check_aborted_status(struct se_cmd *, int);
 int	transport_send_check_condition_and_sense(struct se_cmd *, u8, int);
 
-void	target_get_sess_cmd(struct se_session *, struct se_cmd *);
+void	target_get_sess_cmd(struct se_session *, struct se_cmd *, bool);
 int	target_put_sess_cmd(struct se_session *, struct se_cmd *);
 void	target_splice_sess_cmd_list(struct se_session *);
 void	target_wait_for_sess_cmds(struct se_session *, int);