target: Release COMPARE_AND_WRITE mutex in generic failure path

This patch adds a extra check for SCF_COMPARE_AND_WRITE within
transport_generic_request_failure() to invoke the callback for
compare_and_write_callback() or compare_and_write_done(), in
order to release se_dev->caw_mutex from the generic failure
path.

It also adds to checks within compare_and_write_callback() to
avoid processing when transport_generic_request_failure() occurs
early enough that cmd->t_data_sg or cmd->t_bidi_data_sg have not
been setup yet, nor se_dev->caw_mutex obtained from within
sbc_compare_and_write().

v4 changes:
 - Add explicit check for cmd->transport_complete_callback in
   transport_generic_request_failure() to handle case where
   sbc_compare_and_write()clears callback pointer (Dan Carpenter)

Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Martin Petersen <martin.petersen@oracle.com>
Cc: Chris Mason <chris.mason@fusionio.com>
Cc: James Bottomley <JBottomley@Parallels.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_sbc.c

@@ -372,6 +372,13 @@ static sense_reason_t compare_and_write_callback(struct se_cmd *cmd)
 	sense_reason_t ret = TCM_NO_SENSE;
 	int rc, i;
 
+	/*
+	 * Handle early failure in transport_generic_request_failure(),
+	 * which will not have taken ->caw_mutex yet..
+	 */
+	if (!cmd->t_data_sg || !cmd->t_bidi_data_sg)
+		return TCM_NO_SENSE;
+
 	buf = kzalloc(cmd->data_length, GFP_KERNEL);
 	if (!buf) {
 		pr_err("Unable to allocate compare_and_write buf\n");

drivers/target/target_core_transport.c

@@ -1559,6 +1559,13 @@ void transport_generic_request_failure(struct se_cmd *cmd,
 	 * For SAM Task Attribute emulation for failed struct se_cmd
 	 */
 	transport_complete_task_attr(cmd);
+	/*
+	 * Handle special case for COMPARE_AND_WRITE failure, where the
+	 * callback is expected to drop the per device ->caw_mutex.
+	 */
+	if ((cmd->se_cmd_flags & SCF_COMPARE_AND_WRITE) &&
+	     cmd->transport_complete_callback)
+		cmd->transport_complete_callback(cmd);
 
 	switch (sense_reason) {
 	case TCM_NON_EXISTENT_LUN: