The tgt Task Management Functions (TMF) works in the followings:

- When LLDs queue scsi commands to tgt (scsi_tgt_queue_command), they
need to specify unique 'tag' for each command.

- LLDs call 'int scsi_tgt_tsk_mgmt_request(struct Scsi_Host *host, int,
u64 tag, struct scsi_lun *lun, void *data)'.

- int (* tsk_mgmt_response)(u64 data, int result) is added to
scsi_host_template.

The tag arg in scsi_tgt_queue_command is used only for ABORT_TASK to
identify a single command. Initiators send a command with a unique
tag so LLDs simply call scsi_tgt_queue_command() with it.

With FCP and iSCSI, tag is 32-bit, however, unfortunately, SRP uses
64-bit tag. So we need 64-bit for it.

When an initiator sends a task management request, the LLD calls
scsi_tgt_tsk_mgmt_request. the LLD can use whatever it wants for the
data arg. The data arg is used later as the arg in the
tsk_mgmt_response callback.

tgt core just sends the task management request to user space
(by using TGT_KEVENT_TSK_MGMT_REQ).

In the case of ABORT_TASK, tgtd finds a single command to abort and
sends TGT_UEVENT_CMD_RSP and TGT_UEVENT_TSK_MGMT_RSP events.

tgt core calls eh_abort_handler for TGT_UEVENT_CMD_RSP and then
tsk_mgmt_response for TGT_UEVENT_TSK_MGMT_RSP.

If tgtd fails to find a command to abort, it sends only
TGT_UEVENT_TSK_MGMT_RSP event (no TGT_UEVENT_CMD_RSP event).

In the case of the rests task management functions (like
ABORT_TASK_SET), tgt needs to abort multiple commands. Thus, tgtd
finds multiple commands to abort and sends multiple TGT_UEVENT_CMD_RSP
events and a single TGT_UEVENT_TSK_MGMT_RSP event. tgt core calls
eh_abort_handler multiple times and tsk_mgmt_response once.

eh_abort_handler enables LLDs to safely free resource related with a
command to abort.

Note that when tgtd finds that a TGT_KEVENT_TSK_MGMT_REQ event tries
to abort commands between in TGT_UEVENT_CMD_RSP and
TGT_KEVENT_CMD_DONE states (that is, tgt calls something like
bio_map_user for the commands), tgtd gives up. After all, we cannot
abort such commands. For example, possibly, The commands already touch
page cache. In this case, tgtd waits for the completion of the commands
(TGT_KEVENT_CMD_DONE) and sends TGT_UEVENT_TSK_MGMT_RSP event with an
appropriate result value.
